Part I. What's new in Spring Integration 2.0

For those who are already familiar with Spring Integration, this chapter provides a brief overview of
the new features of version 2.0.

1. What's new in Spring Integration 2.0?

1.1 Spring 3 support

Spring Integration 2.0 is built on top of Spring 3.0.5 and makes many of its features available to our users.

1.1.1 Support for the Spring Expression Language (SpEL)

You can now use SpEL expressions within the transformer, router, filter,
splitter, aggregator, service-activator, header-enricher, and many more elements of the
Spring Integration core namespace as well as various adapters.
There are many samples provided throughout this manual.

1.1.3 TaskScheduler and Trigger

Spring 3.0 defines two new strategies related to scheduling: TaskScheduler and Trigger
Spring Integration (which uses a lot of scheduling) now builds upon these. In fact, Spring Integration 1.0
had originally defined some of the components (e.g. CronTrigger) that have now been migrated into Spring 3.0's
core API. Now, you can benefit from reusing the same components within the entire Application Context (not just
Spring Integration configuration). Configuration of Spring Integration Pollers has been greatly simplified
as well by providing attributes for directly configuring rates, delays, cron expressions, and trigger references.
See Section 3.2, “Channel Adapter” for sample configurations.

1.1.4 RestTemplate and HttpMessageConverter

Our outbound HTTP adapters now delegate to Spring's RestTemplate for executing the HTTP request and handling its response.
This also means that you can reuse any custom HttpMessageConverter implementations.
See Section 13.3, “Http Outbound Gateway” for more details.

1.2 Enterprise Integration Pattern Additions

1.2.1 Message History

We now provide support for the Message History pattern
allowing you to keep track of all traversed components, including the name of each channel and endpoint as well as the timestamp
of that traversal. See Section 8.2, “Message History” for more details.

1.2.3 Claim Check

We have added an implementation of the Claim Check pattern.
The idea behind the Claim Check pattern is that you can exchange a Message payload for a "claim ticket" and vice-versa.
This allows you to reduce bandwidth and/or avoid potential security issues when sending Messages across channels.
See Section 6.3, “Claim Check” for more details.

1.3.2 Twitter Adapters

Twitter adapters provides support for sending and receiving Twitter Status
updates as well as Direct Messages. You can also perform Twitter Searches with an inbound Channel Adapter.
See Chapter 22, Twitter Adapter for more details.

1.4.3 JSON Transformers

1.4.4 Serialization Transformers

These symmetrical transformers convert payload objects to and from byte arrays.
They also support the Serializer and Deserializer strategy interfaces that have been added as of Spring 3.0.5.
See Section 6.1, “Transformer” for more details.

1.5 Framework Refactoring

The core API went through some significant refactoring to make it simpler and more usable. Although we anticipate
that the impact to the end user should be minimal, please read through this document to find what was
changed. Especially, visit Section 5.1.4, “Dynamic Routers” , Section 7.2, “Inbound Messaging Gateways”, Section 13.3, “Http Outbound Gateway”,
Section 4.1, “Message”, and Section 5.4, “Aggregator” for more details. If you are depending directly on some of
the core components (Message, MessageHeaders, MessageChannel, MessageBuilder, etc.), you will notice that you need to
update any import statements. We restructured some packaging to provide the flexibility we needed for extending the
domain model while avoiding any cyclical dependencies (it is a policy of the framework to avoid such "tangles").

1.7 New Spring Integration Samples

With Spring Integration 2.0 we have decoupled the samples from our main release distribution. Please read this blog to get more info
New Spring Integration Samples
We have also created many new samples, including samples for every new Adapter.

1.8 SpringSource Tool Suite Visual Editor for Spring Integration

There is an amazing new visual editor for Spring Integration included within the latest version of SpringSource Tool Suite.
If you are not already using STS 2.5.1, please download it here:
STS

1.9 Upcoming Spring Integration ROO support

We have started working on Spring Integration ROO support, and plan to have a first milestone release soon.
You can follow its development here: Spring Integration Roo Add-on.

Part II. Overview of Spring Integration Framework

Spring Integration provides an extension of the Spring programming model to support the well-known
Enterprise Integration Patterns.
It enables lightweight messaging within Spring-based applications and supports
integration with external systems via declarative adapters.
Those adapters provide a higher-level of abstraction over Spring's support for remoting, messaging, and scheduling.
Spring Integration's primary goal is to provide a simple model for building enterprise integration solutions
while maintaining the separation of concerns that is essential for producing maintainable, testable code.

2. Spring Integration Overview

2.1 Background

One of the key themes of the Spring Framework is inversion of control. In its broadest
sense, this means that the framework handles responsibilities on behalf of the components that are managed within
its context. The components themselves are simplified since they are relieved of those responsibilities. For
example, dependency injection relieves the components of the responsibility of locating or
creating their dependencies. Likewise, aspect-oriented programming relieves business
components of generic cross-cutting concerns by modularizing them into reusable aspects. In each case, the end
result is a system that is easier to test, understand, maintain, and extend.

Furthermore, the Spring framework and portfolio provide a comprehensive programming model for building
enterprise applications. Developers benefit from the consistency of this model and especially the fact that it is
based upon well-established best practices such as programming to interfaces and favoring composition over
inheritance. Spring's simplified abstractions and powerful support libraries boost developer productivity while
simultaneously increasing the level of testability and portability.

Spring Integration is motivated by these same goals and principles. It
extends the Spring programming model into the messaging domain and builds upon Spring's existing enterprise
integration support to provide an even higher level of abstraction. It supports message-driven architectures
where inversion of control applies to runtime concerns, such as when certain business logic
should execute and where the response should be sent. It supports routing and transformation
of messages so that different transports and different data formats can be integrated without impacting
testability. In other words, the messaging and integration concerns are handled by the framework, so business
components are further isolated from the infrastructure and developers are relieved of complex integration
responsibilities.

As an extension of the Spring programming model, Spring Integration provides a wide variety of configuration
options including annotations, XML with namespace support, XML with generic "bean" elements, and of course direct
usage of the underlying API. That API is based upon well-defined strategy interfaces and non-invasive, delegating
adapters. Spring Integration's design is inspired by the recognition of a strong affinity between common patterns
within Spring and the well-known Enterprise Integration Patterns
as described in the book of the same name by Gregor Hohpe and Bobby Woolf (Addison Wesley, 2004). Developers who
have read that book should be immediately comfortable with the Spring Integration concepts and terminology.

The framework should enforce separation of concerns between business logic and
integration logic.

Extension points should be abstract in nature but within well-defined boundaries to promote
reuse and portability.

2.3 Main Components

From the vertical perspective, a layered architecture facilitates separation of concerns,
and interface-based contracts between layers promote loose coupling. Spring-based applications are typically
designed this way, and the Spring framework and portfolio provide a strong foundation for following this best
practice for the full-stack of an enterprise application. Message-driven architectures add a
horizontal perspective, yet these same goals are still relevant. Just as "layered
architecture" is an extremely generic and abstract paradigm, messaging systems typically follow the similarly
abstract "pipes-and-filters" model. The "filters" represent any component that is capable of producing and/or
consuming messages, and the "pipes" transport the messages between filters so that the components themselves
remain loosely-coupled. It is important to note that these two high-level paradigms are not mutually exclusive.
The underlying messaging infrastructure that supports the "pipes" should still be encapsulated in a layer whose
contracts are defined as interfaces. Likewise, the "filters" themselves would typically be managed within a layer
that is logically above the application's service layer, interacting with those services through interfaces much
in the same way that a web-tier would.

2.3.1 Message

In Spring Integration, a Message is a generic wrapper for any Java object combined with metadata used by the
framework while handling that object. It consists of a payload and headers. The payload can be of any type and
the headers hold commonly required information such as id, timestamp, correlation id, and return address. Headers
are also used for passing values to and from connected transports. For example, when creating a Message from a
received File, the file name may be stored in a header to be accessed by downstream components. Likewise, if a
Message's content is ultimately going to be sent by an outbound Mail adapter, the various properties (to, from,
cc, subject, etc.) may be configured as Message header values by an upstream component. Developers can also
store any arbitrary key-value pairs in the headers.

2.3.2 Message Channel

A Message Channel represents the "pipe" of a pipes-and-filters architecture. Producers send Messages to
a channel, and consumers receive Messages from a channel. The Message Channel therefore decouples the
messaging components, and also provides a convenient point for interception and monitoring of Messages.

A Message Channel may follow either Point-to-Point or Publish/Subscribe semantics. With a Point-to-Point
channel, at most one consumer can receive each Message sent to the channel. Publish/Subscribe channels, on the
other hand, will attempt to broadcast each Message to all of its subscribers. Spring Integration supports
both of these.

Whereas "Point-to-Point" and "Publish/Subscribe" define the two options for how many
consumers will ultimately receive each Message, there is another important consideration: should the channel
buffer messages? In Spring Integration, Pollable Channels are capable of buffering
Messages within a queue. The advantage of buffering is that it allows for throttling the inbound Messages and
thereby prevents overloading a consumer. However, as the name suggests, this also adds some complexity, since a
consumer can only receive the Messages from such a channel if a poller is configured. On
the other hand, a consumer connected to a Subscribable Channel is simply Message-driven.
The variety of channel implementations available in Spring Integration will be discussed in detail in
Section 3.1.2, “Message Channel Implementations”.

2.3.3 Message Endpoint

One of the primary goals of Spring Integration is to simplify the development of enterprise integration
solutions through inversion of control. This means that you should not have to implement
consumers and producers directly, and you should not even have to build Messages and invoke send or receive
operations on a Message Channel. Instead, you should be able to focus on your specific domain model with an
implementation based on plain Objects. Then, by providing declarative configuration, you can "connect"
your domain-specific code to the messaging infrastructure provided by Spring Integration. The components
responsible for these connections are Message Endpoints. This does not mean that you will necessarily connect
your existing application code directly. Any real-world enterprise integration solution will require some
amount of code focused upon integration concerns such as routing and
transformation. The important thing is to achieve separation of concerns between such
integration logic and business logic. In other words, as with the Model-View-Controller paradigm for web
applications, the goal should be to provide a thin but dedicated layer that translates inbound requests into
service layer invocations, and then translates service layer return values into outbound replies. The next
section will provide an overview of the Message Endpoint types that handle these responsibilities, and in
upcoming chapters, you will see how Spring Integration's declarative configuration options provide a
non-invasive way to use each of these.

2.4 Message Endpoints

A Message Endpoint represents the "filter" of a pipes-and-filters architecture. As mentioned above, the
endpoint's primary role is to connect application code to the messaging framework and to do so in a non-invasive
manner. In other words, the application code should ideally have no awareness of the Message objects or the
Message Channels. This is similar to the role of a Controller in the MVC paradigm. Just as a Controller handles
HTTP requests, the Message Endpoint handles Messages. Just as Controllers are mapped to URL patterns, Message
Endpoints are mapped to Message Channels. The goal is the same in both cases: isolate application code from the
infrastructure. These concepts are discussed at length along with all of the patterns that follow in the
Enterprise Integration Patterns book. Here, we provide only a
high-level description of the main endpoint types supported by Spring Integration and their roles. The chapters
that follow will elaborate and provide sample code as well as configuration examples.

2.4.1 Transformer

A Message Transformer is responsible for converting a Message's content or structure and returning the modified
Message. Probably the most common type of transformer is one that converts the payload of the Message from one
format to another (e.g. from XML Document to java.lang.String). Similarly, a transformer may be used to add,
remove, or modify the Message's header values.

2.4.2 Filter

A Message Filter determines whether a Message should be passed to an output channel at all. This simply
requires a boolean test method that may check for a particular payload content type, a property value, the
presence of a header, etc. If the Message is accepted, it is sent to the output channel, but if not it will be
dropped (or for a more severe implementation, an Exception could be thrown). Message Filters are often used in
conjunction with a Publish Subscribe channel, where multiple consumers may receive the same Message and use the
filter to narrow down the set of Messages to be processed based on some criteria.

Note

Be careful not to confuse the generic use of "filter" within the Pipes-and-Filters architectural pattern with
this specific endpoint type that selectively narrows down the Messages flowing between two channels. The
Pipes-and-Filters concept of "filter" matches more closely with Spring Integration's Message Endpoint: any
component that can be connected to Message Channel(s) in order to send and/or receive Messages.

2.4.3 Router

A Message Router is responsible for deciding what channel or channels should receive the Message next (if any).
Typically the decision is based upon the Message's content and/or metadata available in the Message Headers.
A Message Router is often used as a dynamic alternative to a statically configured output channel on
a Service Activator or other endpoint capable of sending reply Messages. Likewise, a Message Router provides a
proactive alternative to the reactive Message Filters used by multiple subscribers as described above.

2.4.4 Splitter

A Splitter is another type of Message Endpoint whose responsibility is to accept a Message from its input
channel, split that Message into multiple Messages, and then send each of those to its output channel. This
is typically used for dividing a "composite" payload object into a group of Messages containing the
sub-divided payloads.

2.4.5 Aggregator

Basically a mirror-image of the Splitter, the Aggregator is a type of Message Endpoint that receives multiple
Messages and combines them into a single Message. In fact, Aggregators are often downstream consumers in a
pipeline that includes a Splitter. Technically, the Aggregator is more complex than a Splitter, because it
is required to maintain state (the Messages to-be-aggregated), to decide when the complete group of Messages
is available, and to timeout if necessary. Furthermore, in case of a timeout, the Aggregator needs to know
whether to send the partial results or to discard them to a separate channel. Spring Integration provides
a CompletionStrategy as well as configurable settings for timeout, whether
to send partial results upon timeout, and the discard channel.

2.4.6 Service Activator

A Service Activator is a generic endpoint for connecting a service instance to the messaging system. The
input Message Channel must be configured, and if the service method to be invoked is capable of returning a
value, an output Message Channel may also be provided.

Note

The output channel is optional, since each Message may also provide its own 'Return Address' header. This
same rule applies for all consumer endpoints.

The Service Activator invokes an operation on some service object to process the request Message, extracting
the request Message's payload and converting if necessary (if the method does not expect a Message-typed
parameter). Whenever the service object's method returns a value, that return value will likewise be converted
to a reply Message if necessary (if it's not already a Message). That reply Message is sent to the output
channel. If no output channel has been configured, then the reply will be sent to the channel specified in the
Message's "return address" if available.

2.4.7 Channel Adapter

A Channel Adapter is an endpoint that connects a Message Channel to some other system or transport. Channel
Adapters may be either inbound or outbound. Typically, the Channel Adapter will do some mapping between the
Message and whatever object or resource is received-from or sent-to the other system (File, HTTP Request, JMS
Message, etc). Depending on the transport, the Channel Adapter may also populate or extract Message header
values. Spring Integration provides a number of Channel Adapters, and they will be described in upcoming
chapters.

An inbound "Channel Adapter" endpoint connects a source system to a MessageChannel.

An outbound "Channel Adapter" endpoint connects a MessageChannel to a target system.

Part III. Core Messaging

This section covers all aspects of the core messaging API in Spring Integration.
Here you will learn about Messages, Message Channels, and Message Endpoints.
Many of the Enterprise Integration Patterns are covered here as well,
such as Filters, Routers, Transformers, Service-Activators, Splitters, and Aggregators.
The section also contains material about System Management, including the Control Bus and Message History support.

3. Messaging Channels

3.1 Message Channels

While the Message plays the crucial role of encapsulating data, it is the
MessageChannel that decouples message producers from message consumers.

3.1.1 The MessageChannel Interface

When sending a message, the return value will be true if the message is sent successfully.
If the send call times out or is interrupted, then it will return false.

PollableChannel

Since Message Channels may or may not buffer Messages (as discussed in the overview), there are two
sub-interfaces defining the buffering (pollable) and non-buffering (subscribable) channel behavior. Here is the
definition of PollableChannel.

Similar to the send methods, when receiving a message, the return value will be null in the
case of a timeout or interrupt.

SubscribableChannel

The SubscribableChannel base interface is implemented by channels that send
Messages directly to their subscribed MessageHandlers. Therefore, they do not
provide receive methods for polling, but instead define methods for managing those subscribers:

3.1.2 Message Channel Implementations

Spring Integration provides several different Message Channel implementations. Each is briefly described in the
sections below.

PublishSubscribeChannel

The PublishSubscribeChannel implementation broadcasts any Message
sent to it to all of its subscribed handlers. This is most often used for sending
Event Messages whose primary role is notification as opposed to
Document Messages which are generally intended to be processed by
a single handler. Note that the PublishSubscribeChannel is
intended for sending only. Since it broadcasts to its subscribers directly when its
send(Message) method is invoked, consumers cannot poll for
Messages (it does not implement PollableChannel and
therefore has no receive() method). Instead, any subscriber
must be a MessageHandler itself, and the subscriber's
handleMessage(Message) method will be invoked in turn.

QueueChannel

The QueueChannel implementation wraps a queue. Unlike the
PublishSubscribeChannel, the QueueChannel has point-to-point
semantics. In other words, even if the channel has multiple consumers, only one of them should receive any
Message sent to that channel. It provides a default no-argument constructor (providing an essentially unbounded
capacity of Integer.MAX_VALUE) as well as a constructor that accepts the queue capacity:

public QueueChannel(int capacity)

A channel that has not reached its capacity limit will store messages in its internal queue, and the
send() method will return immediately even if no receiver is ready to handle the
message. If the queue has reached capacity, then the sender will block until room is available. Or, if using
the send call that accepts a timeout, it will block until either room is available or the timeout period
elapses, whichever occurs first. Likewise, a receive call will return immediately if a message is available
on the queue, but if the queue is empty, then a receive call may block until either a message is available
or the timeout elapses. In either case, it is possible to force an immediate return regardless of the
queue's state by passing a timeout value of 0. Note however, that calls to the no-arg versions of
send() and receive() will block indefinitely.

PriorityChannel

Whereas the QueueChannel enforces first-in/first-out (FIFO) ordering, the
PriorityChannel is an alternative implementation that allows for messages
to be ordered within the channel based upon a priority. By default the priority is determined by the
'priority' header within each message. However, for custom priority determination
logic, a comparator of type Comparator<Message<?>> can be provided
to the PriorityChannel's constructor.

RendezvousChannel

The RendezvousChannel enables a "direct-handoff" scenario where a sender will block
until another party invokes the channel's receive() method or vice-versa. Internally,
this implementation is quite similar to the QueueChannel except that it uses a
SynchronousQueue (a zero-capacity implementation of
BlockingQueue). This works well in situations where the sender and receiver are
operating in different threads but simply dropping the message in a queue asynchronously is not appropriate.
In other words, with a RendezvousChannel at least the sender knows that some receiver
has accepted the message, whereas with a QueueChannel, the message would have been
stored to the internal queue and potentially never received.

Tip

Keep in mind that all of these queue-based channels are storing messages in-memory only by default.
When persistence is required, you can either provide a 'message-store' attribute within the 'queue'
element to reference a persistent MessageStore implementation, or you can replace the local channel
with one that is backed by a persistent broker, such as a JMS-backed channel or Channel Adapter.
The latter option allows you to take advantage of any JMS provider's
implementation for message persistence, and it will be discussed in Chapter 18, JMS Support. However, when
buffering in a queue is not necessary, the simplest approach is to rely upon the
DirectChannel discussed next.

The RendezvousChannel is also useful for implementing request-reply
operations. The sender can create a temporary, anonymous instance of RendezvousChannel
which it then sets as the 'replyChannel' header when building a Message. After sending that Message, the sender
can immediately call receive (optionally providing a timeout value) in order to block while waiting for a reply
Message. This is very similar to the implementation used internally by many of Spring Integration's
request-reply components.

DirectChannel

The DirectChannel has point-to-point semantics but otherwise is more similar to the
PublishSubscribeChannel than any of the queue-based channel implementations described
above. It implements the SubscribableChannel interface instead of the
PollableChannel interface, so it dispatches Messages directly to a subscriber.
As a point-to-point channel, however, it differs from the PublishSubscribeChannel in
that it will only send each Message to a single subscribed
MessageHandler.

In addition to being the simplest point-to-point channel option, one of its most important features is that
it enables a single thread to perform the operations on "both sides" of the channel. For example, if a handler
is subscribed to a DirectChannel, then sending a Message to that channel will trigger
invocation of that handler's handleMessage(Message) method directly in the
sender's thread, before the send() method invocation can return.

The key motivation for providing a channel implementation with this behavior is to support transactions that
must span across the channel while still benefiting from the abstraction and loose coupling that the channel
provides. If the send call is invoked within the scope of a transaction, then the outcome of the handler's
invocation (e.g. updating a database record) will play a role in determining the ultimate result of that
transaction (commit or rollback).

Note

Since the DirectChannel is the simplest option and does not add any additional
overhead that would be required for scheduling and managing the threads of a poller, it is the default
channel type within Spring Integration. The general idea is to define the channels for an application and
then to consider which of those need to provide buffering or to throttle input, and then modify those to
be queue-based PollableChannels. Likewise, if a channel needs to broadcast
messages, it should not be a DirectChannel but rather a
PublishSubscribeChannel. Below you will see how each of these can be configured.

The DirectChannel internally delegates to a Message Dispatcher to invoke its
subscribed Message Handlers, and that dispatcher can have a load-balancing strategy. The load-balancer
determines how invocations will be ordered in the case that there are multiple handlers subscribed to the
same channel. When using the namespace support described below, the default strategy is
"round-robin" which essentially load-balances across the handlers in rotation.

Note

The "round-robin" strategy is currently the only implementation available out-of-the-box in Spring
Integration. Other strategy implementations may be added in future versions.

The load-balancer also works in combination with a boolean failover property.
If the "failover" value is true (the default), then the dispatcher will fall back to any subsequent
handlers as necessary when preceding handlers throw Exceptions. The order is determined by an optional
order value defined on the handlers themselves or, if no such value exists, the order in which the
handlers are subscribed.

If a certain situation requires that the dispatcher always try to invoke the first handler, then
fallback in the same fixed order sequence every time an error occurs, no load-balancing strategy should
be provided. In other words, the dispatcher still supports the failover boolean property even when no
load-balancing is enabled. Without load-balancing, however, the invocation of handlers will always begin
with the first according to their order. For example, this approach works well when there is a clear
definition of primary, secondary, tertiary, and so on. When using the namespace support, the "order"
attribute on any endpoint will determine that order.

Note

Keep in mind that load-balancing and failover only apply when a channel has more than one
subscribed Message Handler. When using the namespace support, this means that more than one
endpoint shares the same channel reference in the "input-channel" attribute.

ExecutorChannel

The ExecutorChannel is a point-to-point channel that supports
the same dispatcher configuration as DirectChannel (load-balancing strategy
and the failover boolean property). The key difference between these two dispatching channel types
is that the ExecutorChannel delegates to an instance of
TaskExecutor to perform the dispatch. This means that the send method
typically will not block, but it also means that the handler invocation may not occur in the sender's
thread. It therefore does not support transactions spanning the sender and receiving
handler.

Tip

Note that there are occasions where the sender may block. For example, when using a
TaskExecutor with a rejection-policy that throttles back on the client (such as the
ThreadPoolExecutor.CallerRunsPolicy), the sender's thread will execute
the method directly anytime the thread pool is at its maximum capacity and the
executor's work queue is full. Since that situation would only occur in a non-predictable
way, that obviously cannot be relied upon for transactions.

Scoped Channel

Spring Integration 1.0 provided a ThreadLocalChannel implementation, but that has been removed as of 2.0. Now, there is a more general way for handling the same requirement by simply adding a "scope" attribute to a channel. The value of the attribute can be any name of a Scope that is available within the context. For example, in a web environment, certain Scopes are available, and any custom Scope implementations can be registered with the context. Here's an example of a ThreadLocal-based scope being applied to a channel, including the registration of the Scope itself.

The channel above also delegates to a queue internally, but the channel is bound
to the current thread, so the contents of the queue are as well. That way the thread that
sends to the channel will later be able to receive those same Messages, but no other thread
would be able to access them. While thread-scoped channels are rarely needed, they can be
useful in situations where DirectChannels are being used to enforce a
single thread of operation but any reply Messages should be sent to a "terminal" channel.
If that terminal channel is thread-scoped, the original sending thread can collect its replies from it.

Now, since any channel can be scoped, you can define your own scopes in addition to Thread Local.

3.1.3 Channel Interceptors

One of the advantages of a messaging architecture is the ability to provide common behavior and capture
meaningful information about the messages passing through the system in a non-invasive way. Since the
Messages are being sent to and received from
MessageChannels, those channels provide an opportunity for intercepting
the send and receive operations. The ChannelInterceptor strategy interface
provides methods for each of those operations:

After implementing the interface, registering the interceptor with a channel is just a matter of calling:

channel.addInterceptor(someChannelInterceptor);

The methods that return a Message instance can be used for transforming the Message or can return 'null'
to prevent further processing (of course, any of the methods can throw a RuntimeException). Also, the
preReceive method can return 'false' to prevent the receive
operation from proceeding.

Note

Keep in mind that receive() calls are only relevant for
PollableChannels. In fact the
SubscribableChannel interface does not even define a
receive() method. The reason for this is that when a Message is sent to a
SubscribableChannel it will be sent directly to one or more subscribers
depending on the type of channel (e.g. a PublishSubscribeChannel sends to all of its subscribers). Therefore,
the preReceive(..) and postReceive(..) interceptor methods
are only invoked when the interceptor is applied to a PollableChannel.

Spring Integration also provides an implementation of the
Wire Tap pattern.
It is a simple interceptor that sends the Message to another channel without otherwise altering the
existing flow. It can be very useful for debugging and monitoring. An example is shown in
the section called “Wire Tap”.

Because it is rarely necessary to implement all of the interceptor methods, a
ChannelInterceptorAdapter class is also available for sub-classing. It provides no-op
methods (the void method is empty, the Message returning methods
return the Message as-is, and the boolean method returns true).
Therefore, it is often easiest to extend that class and just implement the method(s) that you need as in the
following example.

The order of invocation for the interceptor methods depends on the type of channel. As described above,
the queue-based channels are the only ones where the receive method is intercepted in the first place.
Additionally, the relationship between send and receive interception depends on the timing of separate
sender and receiver threads. For example, if a receiver is already blocked while waiting for a message
the order could be: preSend, preReceive, postReceive, postSend. However, if a receiver polls after the
sender has placed a message on the channel and already returned, the order would be: preSend, postSend,
(some-time-elapses) preReceive, postReceive. The time that elapses in such a case depends on a number
of factors and is therefore generally unpredictable (in fact, the receive may never happen!).
Obviously, the type of queue also plays a role (e.g. rendezvous vs. priority). The bottom line is that
you cannot rely on the order beyond the fact that preSend will precede postSend and preReceive will
precede postReceive.

3.1.4 MessagingTemplate

As you will see when the endpoints and their various configuration options are introduced, Spring Integration
provides a foundation for messaging components that enables non-invasive invocation of your application code
from the messaging system. However, sometimes it is necessary to invoke the messaging system
from your application code. For convenience when implementing such use-cases, Spring
Integration provides a MessagingTemplate that supports a variety of operations across
the Message Channels, including request/reply scenarios. For example, it is possible to send a request
and wait for a reply.

In that example, a temporary anonymous channel would be created internally by the template. The
'sendTimeout' and 'receiveTimeout' properties may also be set on the template, and other exchange
types are also supported.

DirectChannel Configuration

As mentioned above, DirectChannel is the default type.

<channelid="directChannel"/>

A default channel will have a round-robin load-balancer and will also have
failover enabled (See the discussion in the section called “DirectChannel”
for more detail). To disable one or both of these, add a <dispatcher/> sub-element and
configure the attributes:

Datatype Channel Configuration

There are times when a consumer can only process a particular type of payload and you need to therefore ensure the payload type of input Messages.
Of course the first thing that comes to mind is Message Filter. However all that Message Filter will do is filter out Messages that are not compliant with
the requirements of the consumer. Another way would be to use a Content Based Router and route Messages with non-compliant data-types to specific
Transformers to enforce transformation/conversion to the required data-type. This of course would work, but a simpler way of accomplishing the
same thing is to apply the Datatype Channel pattern.
You can use separate Datatype Channels for each specific payload data-type.

To create a Datatype Channel that only
accepts messages containing a certain payload type, provide the fully-qualified class name in the
channel element's datatype attribute:

<int:channelid="numberChannel"datatype="java.lang.Number"/>

Note that the type check passes for any type that is assignable to the channel's
datatype. In other words, the "numberChannel" above would accept messages whose payload is
java.lang.Integer or java.lang.Double. Multiple types can be
provided as a comma-delimited list:

So the 'numberChannel' above will only accept Messages with a data-type of java.lang.Number. But what happens
if the payload of the Message is not of the required type? It depends on whether you have defined a bean named "integrationConversionService"
that is an instance of Spring's Conversion Service.
If not, then an Exception would be thrown immediately, but if you do have an "integrationConversionService" bean defined, it will be used
in an attempt to convert the Message's payload to the acceptable type.

You can even register custom converters. For example, let's say you are sending a Message with a String payload to the 'numberChannel' we configured above.

When the 'converter' element is parsed, it will create the "integrationConversionService" bean on-demand if one is not already defined.
With that Converter in place, the send operation would now be successful since the Datatype Channel will use that Converter to convert the String
payload to an Integer.

QueueChannel Configuration

To create a QueueChannel, use the <queue/> sub-element.
You may specify the channel's capacity:

<channelid="queueChannel"><queuecapacity="25"/></channel>

Note

If you do not provide a value for the 'capacity' attribute on this <queue/> sub-element,
the resulting queue will be unbounded. To avoid issues such as OutOfMemoryErrors, it is highly
recommended to set an explicit value for a bounded queue.

Persistent QueueChannel Configuration

Since a QueueChannel provides the capability to buffer Messages, but does so in-memory only
by default, it also introduces a possibility that Messages could be lost in the event of a system failure. To
mitigate this risk, a QueueChannel may be backed by a persistent implementation of the
MessageGroupStore strategy interface. For more details on MessageGroupStore
and MessageStore see Section 17.4, “Message Store”.

When a QueueChannel receives a Message, it will add it to the Message Store, and when a Message
is polled from a QueueChannel, it is removed from the Message Store.

By default any QueueChannel only stores its Messages in an in-memory Queue
and can therefore lead to the lost message scenario mentioned above. However Spring Integration
provides a JdbcMessageStore to allow a QueueChannel to be backed by an RDBMS.

You can configure a Message Store for any QueueChannel by adding the
message-store attribute as shown in the next example.

The above example also shows that JdbcMessageStore can be configured with the namespace support
provided by the Spring Integration JDBC module. All you need to do is inject any javax.sql.DataSource
instance. The Spring Integration JDBC module also provides schema DDL for most popular databases. These schemas are located in
the org.springframework.integration.jdbc package of that module (spring-integration-jdbc).

Important

One important feature is that with any transactional persistent store (e.g., JdbcMessageStore), as long as the poller has a transaction configured,
a Message removed from the store will only be permanently removed if the transaction completes
successfully, otherwise the transaction will roll back and the Message will not be lost.

Many other implementations of the Message Store will be available as the growing number of Spring projects
related to "NoSQL" data stores provide the underlying support. Of course, you can always provide your own implementation
of the MessageGroupStore interface if you cannot find one that meets your particular needs.

PublishSubscribeChannel Configuration

To create a PublishSubscribeChannel, use the <publish-subscribe-channel/> element.
When using this element, you can also specify the task-executor used for publishing
Messages (if none is specified it simply publishes in the sender's thread):

If you are providing a Resequencer or Aggregator downstream
from a PublishSubscribeChannel, then you can set the 'apply-sequence' property
on the channel to true. That will indicate that the channel should set the sequence-size
and sequence-number Message headers as well as the correlation id prior to passing the Messages along.
For example, if there are 5 subscribers, the sequence-size would be set to 5, and the Messages would
have sequence-number header values ranging from 1 to 5.

<publish-subscribe-channelid="pubsubChannel"apply-sequence="true"/>

Note

The apply-sequence value is false by default so that a Publish Subscribe Channel
can send the exact same Message instances to multiple outbound channels. Since Spring Integration
enforces immutability of the payload and header references, the channel creates new Message
instances with the same payload reference but different header values when the flag is set to
true.

ExecutorChannel

To create an ExecutorChannel, add the <dispatcher> sub-element along
with a task-executor attribute. Its value can reference any TaskExecutor
within the context. For example, this enables configuration of a thread-pool for dispatching messages
to subscribed handlers. As mentioned above, this does break the "single-threaded" execution context
between sender and receiver so that any active transaction context will not be shared by the invocation
of the handler (i.e. the handler may throw an Exception, but the send invocation has already returned
successfully).

The load-balancer and failover options are also both available on the <dispatcher/> sub-element
as described above in the section called “DirectChannel Configuration”. The same defaults
apply as well. So, the channel will have a round-robin load-balancing strategy with failover
enabled unless explicit configuration is provided for one or both of those attributes.

PriorityChannel Configuration

To create a PriorityChannel, use the <priority-queue/> sub-element:

<channelid="priorityChannel"><priority-queuecapacity="20"/></channel>

By default, the channel will consult the priority header of the
message. However, a custom Comparator reference may be
provided instead. Also, note that the PriorityChannel (like the other types)
does support the datatype attribute. As with the QueueChannel, it also supports a capacity attribute.
The following example demonstrates all of these:

RendezvousChannel Configuration

A RendezvousChannel is created when the queue sub-element is
a <rendezvous-queue>. It does not provide any additional configuration options to
those described above, and its queue does not accept any capacity value since it is a
0-capacity direct handoff queue.

<channelid="rendezvousChannel"/><rendezvous-queue/></channel>

Scoped Channel Configuration

Any channel can be configured with a "scope" attribute.

<channelid="threadLocalChannel"scope="thread"/>

Channel Interceptor Configuration

Message channels may also have interceptors as described in Section 3.1.3, “Channel Interceptors”. The
<interceptors/> sub-element can be added within <channel/> (or the more specific element
types). Provide the ref attribute to reference any Spring-managed object that implements the
ChannelInterceptor interface:

In general, it is a good idea to define the interceptor implementations in a separate location since they
usually provide common behavior that can be reused across multiple channels.

Global Channel Interceptor Configuration

Channel Interceptors provide a clean and concise way of applying cross-cutting behavior per individual channel.
If the same behavior should be applied on multiple channels, configuring the same set of interceptors for
each channel would not be the most efficient way. To avoid repeated configuration while
also enabling interceptors to apply to multiple channels, Spring Integration provides
Global Interceptors.
Look at the example below:

Each <channel-interceptor/> element allows you to define a global interceptor which will be applied on all
channels that match any patterns defined via the pattern attribute. In the above case the
global interceptor will be applied on the
'foo' channel and all other channels that begin with 'bar' or 'input'.
The order attribute allows you to manage where this interceptor will be injected if there
are multiple interceptors on a given channel.
For example, channel 'inputChannel' could have individual interceptors configured locally (see below):

A reasonable question is how will a global interceptor be injected in relation to other interceptors
configured locally or through other global interceptor definitions? The current implementation provides
a very simple mechanism for defining the order of interceptor execution.
A positive number in the order attribute will ensure interceptor injection
after any existing interceptors and a negative number will ensure that the interceptor is injected before
existing interceptors.
This means that in the above example, the global interceptor will be injected AFTER
(since its order is greater than 0)
the 'wire-tap' interceptor configured locally. If there were another global interceptor with a matching
pattern, its order would be determined by comparing the values of the order attribute.
To inject a global interceptor BEFORE the existing interceptors, use a negative value for the order attribute.

Note

Note that both the order and pattern attributes are optional. The default value
for order will be 0 and for pattern, the default is '*' (to match all channels).

Wire Tap

As mentioned above, Spring Integration provides a simple Wire Tap interceptor out of
the box. You can configure a Wire Tap on any channel within an <interceptors/> element.
This is especially useful for debugging, and can be used in conjunction with Spring Integration's logging
Channel Adapter as follows:

The 'logging-channel-adapter' also accepts an 'expression' attribute so that you can evaluate
a SpEL expression against 'payload' and/or 'headers' variables. Alternatively, to simply log
the full Message toString() result, provide a value of "true" for the 'log-full-message' attribute.
That is false by default so that only the payload is logged. Setting that to
true enables logging of all headers in addition to the payload. The 'expression'
option does provide the most flexibility, however (e.g. expression="payload.user.name").

A little more on Wire Tap

One of the common misconceptions about the wire tap and other similar components (Section 26.1, “Message Publishing Configuration”)
is that they are automatically asynchronous in nature. Wire-tap as a component is not
invoked asynchronously be default. Instead, Spring Integration focuses on a single unified
approach to configuring asynchronous behavior: the Message Channel.
What makes certain parts of the message flow sync or async
is the type of Message Channel that has been configured within that flow. That
is one of the primary benefits of the Message Channel abstraction.
From the inception of the framework, we have always emphasized the need and the value of the
Message Channel as a first-class citizen of the framework. It is not
just an internal, implicit realization of the EIP pattern, it is fully exposed as a configurable
component to the end user.
So, the Wire-tap component is ONLY responsible for performing the following 3 tasks:

intercept a message flow by tapping into a channel (e.g., channelA)

grab each message

send the message to another channel (e.g., channelB)

It is essentially a variation of the Bridge, but it is encapsulated within a channel definition
(and hence easier to enable and disable without disrupting a flow). Also, unlike the bridge, it
basically forks another message flow. Is that flow synchronous or
asynchronous? The answer simply depends on the type of Message Channel
that 'channelB' is. And, now you know that we have: Direct Channel,
Pollable Channel, and Executor Channel as options.
The last two do break the thread boundary making communication via such channels
asynchronous simply because the dispatching of the message from that channel
to its subscribed handlers happens on a different thread than the one used to send the message to that
channel. That is what is going to make your wire-tap flow sync or async.
It is consistent with other components within the framework (e.g., Message Publisher) and actually
brings a level of consistency and simplicity by sparing you from worrying in advance (other than writing
thread safe code) whether a particular piece of code should be implemented as sync or
async. The actual wiring of two pieces of code (component A and component B) via
Message Channel is what makes their collaboration sync or
async. You may even want to change from sync to
async in the future and Message Channel is what's going
to allow you to do it swiftly without ever touching the code.

One final point regarding the Wire Tap is that, despite the rationale provided above for not
being async be default, one should keep in mind it is usually desirable to hand off the Message as
soon as possible. Therefore, it would be quite common to use an asynchronous channel option as the
wire-tap's outbound channel. Nonetheless, another reason that we do not enforce asynchronous behavior
by default is that you might not want to break a transactional boundary. Perhaps you are using the Wire Tap
for auditing purposes, and you DO want the audit Messages to be sent within the original transaction.
As an example, you might connect the wire-tap to a JMS outbound-channel-adapter. That way, you get the
best of both worlds: 1) the sending of a JMS Message can occur within the transaction while
2) it is still a "fire-and-forget" action thereby preventing any noticeable delay in the main message flow.

Global Wire Tap Configuration

It is possible to configure a global wire tap as a special case of the Global Channel Interceptor. Simply configure a top level wire-tap element. Now, in addition to the normal wire-tap namespace support, the pattern and order attributes are supported and work in exactly the same way as with the channel-interceptor

A global wire tap provides a convenient way to configure a single channel wire tap externally without modifying the existing channel configuration. Simply set the pattern attribute to the target channel name. For example, This technique may be used to configure a test case to verify messages on a channel.

3.1.6 Special Channels

If namespace support is enabled, there are two special channels defined within the application context by default:
errorChannel and nullChannel. The 'nullChannel' acts like /dev/null,
simply logging any Message sent to it at DEBUG level and returning immediately. Any time you face channel
resolution errors for a reply that you don't care about, you can set the affected component's output-channel attribute
to 'nullChannel' (the name 'nullChannel' is reserved within the application context). The 'errorChannel' is
used internally for sending error messages and may be overridden with a custom configuration. This is
discussed in greater detail in Section B.4, “Error Handling”.

3.2 Channel Adapter

A Channel Adapter is a Message Endpoint that enables connecting a single sender or receiver to a Message Channel.
Spring Integration provides a number of adapters out of the box to support various transports, such as JMS, File,
HTTP, Web Services, Mail, and more. Those will be discussed in upcoming chapters of this reference guide. However, this
chapter focuses on the simple but flexible Method-invoking Channel Adapter support. There are both inbound and
outbound adapters, and each may be configured with XML elements provided in the core namespace. These provide an
easy way to extend Spring Integration as long as you have a method that can be invoked as either a source or
destination.

3.2.1 Configuring Inbound Channel Adapter

An "inbound-channel-adapter" element can invoke any method on a Spring-managed Object and send a non-null return
value to a MessageChannel after converting it to a Message.
When the adapter's subscription is activated, a poller will attempt to receive messages from the source. The
poller will be scheduled with the TaskScheduler according to the provided
configuration. To configure the polling interval or cron expression for an individual channel-adapter,
provide a 'poller' element with one of the scheduling attributes, such as 'fixed-rate' or 'cron'.

Some inbound-channel-adapter types are backed by a SourcePollingChannelAdapter which
means they contain Poller configuration which will poll the MessageSource (invoke a custom method
which produces the value that becomes a Message payload) based on the configuration
specified in the Poller.

In the the first configuration the polling task will be invoked once per poll and during such task (poll)
the method (which results in the production of the Message) will be invoked once based on the
max-messages-per-poll attribute value. In the second configuration the polling task will be invoked
10 times per poll or until it returns 'null' thus possibly producing 10 Messages per poll while each poll happens
at 1 second intervals.
However what if the configuration looks like this:

<pollerfixed-rate="1000"/>

Note there is no max-messages-per-poll specified. As you'll learn later the identical poller configuration
in the PollingConsumer (e.g., service-activator, filter, router etc.) would have a default
value of -1 for max-messages-per-poll which means "execute poling task non-stop unless polling method
returns null (e.g., no more Messages in the QueueChannel)" and then sleep for 1 second.

However in the SourcePollingChannelAdapter it is a bit different.
The default value for max-messages-per-poll will be set to 1 by default unless you explicitly set it to
a negative value (e.g., -1). It is done so to make sure that poller can react to a LifeCycle events (e.g., start/stop)
and prevent it from potentially spinning in the infinite loop if the implementation of the custom
method of the MessageSource has a potential to never return null and happened to be non-interruptible.

However if you are sure that your method can return null and you need the behavior where you want to poll
for as many sources as available per each poll, then you should explicitly set max-messages-per-poll
to negative value.

<pollermax-messages-per-poll="-1"fixed-rate="1000"/>

3.2.2 Configuring Outbound Channel Adapter

An "outbound-channel-adapter" element can also connect a MessageChannel to any POJO consumer
method that should be invoked with the payload of Messages sent to that channel.

Using a "ref" attribute is generally recommended if the POJO consumer implementation can be reused
in other <outbound-channel-adapter> definitions. However if the consumer implementation
is only referenced by a single definition of the <outbound-channel-adapter>, you
can define it as inner bean:

Using both the "ref" attribute and an inner handler definition in the same <outbound-channel-adapter>
configuration is not allowed as it creates an ambiguous condition. Such a configuration will result in an Exception
being thrown.

Any Channel Adapter can be created without a "channel" reference in which case it will implicitly create an
instance of DirectChannel. The created channel's name will match the "id" attribute
of the <inbound-channel-adapter/> or <outbound-channel-adapter> element. Therefore, if the "channel"
is not provided, the "id" is required.

3.3 Messaging Bridge

3.3.1 Introduction

A Messaging Bridge is a relatively trivial endpoint that simply connects two Message Channels or Channel
Adapters. For example, you may want to connect a PollableChannel to a
SubscribableChannel so that the subscribing endpoints do not have to worry
about any polling configuration. Instead, the Messaging Bridge provides the polling configuration.

By providing an intermediary poller between two channels, a Messaging Bridge can be used to throttle inbound
Messages. The poller's trigger will determine the rate at which messages arrive on the second channel, and the
poller's "maxMessagesPerPoll" property will enforce a limit on the throughput.

Another valid use for a Messaging Bridge is to connect two different systems. In such a scenario, Spring
Integration's role would be limited to making the connection between these systems and managing a poller
if necessary. It is probably more common to have at least a Transformer between the
two systems to translate between their formats, and in that case, the channels would be provided as the
'input-channel' and 'output-channel' of a Transformer endpoint. If data format translation is not required,
the Messaging Bridge may indeed be sufficient.

3.3.2 Configuring Bridge

The <bridge> element is used to create a Messaging Bridge between two Message Channels or Channel Adapters.
Simply provide the "input-channel" and "output-channel" attributes:

<bridgeinput-channel="input"output-channel="output"/>

As mentioned above, a common use case for the Messaging Bridge is to connect a
PollableChannel to a SubscribableChannel, and when
performing this role, the Messaging Bridge may also serve as a throttler:

Of course, the configuration would be similar for other (potentially more useful) Channel Adapter bridges, such
as File to JMS, or Mail to File. The various Channel Adapters will be discussed in upcoming chapters.

Note

If no 'output-channel' is defined on a bridge, the reply channel provided by the inbound Message will
be used, if available. If neither output or reply channel is available, an Exception will be thrown.

4. Message Construction

4.1 Message

The Spring Integration Message is a generic container for data. Any object can
be provided as the payload, and each Message also includes headers containing
user-extensible properties as key-value pairs.

4.1.1 The Message Interface

The Message is obviously a very important part of the API. By encapsulating the
data in a generic wrapper, the messaging system can pass it around without any knowledge of the data's type. As
an application evolves to support new types, or when the types themselves are modified and/or extended, the
messaging system will not be affected by such changes. On the other hand, when some component in the messaging
system does require access to information about the Message,
such metadata can typically be stored to and retrieved from the metadata in the Message Headers.

4.1.2 Message Headers

Just as Spring Integration allows any Object to be used as the payload of a Message, it also supports any Object
types as header values. In fact, the MessageHeaders class implements the
java.util.Map interface:

Even though the MessageHeaders implements Map, it is effectively a read-only implementation. Any attempt to
put a value in the Map will result in an UnsupportedOperationException.
The same applies for remove and clear. Since Messages may be passed to
multiple consumers, the structure of the Map cannot be modified. Likewise, the Message's payload Object can not
be set after the initial creation. However, the mutability of the header values themselves
(or the payload Object) is intentionally left as a decision for the framework user.

As an implementation of Map, the headers can obviously be retrieved by calling get(..)
with the name of the header. Alternatively, you can provide the expected Class as an
additional parameter. Even better, when retrieving one of the pre-defined values, convenient getters are
available. Here is an example of each of these three options:

Notice that this implementation takes advantage of the fact that the GenericMessage
base class is parameterized. Therefore, as shown in both examples, no casting is necessary when retrieving
the Message payload Object.

4.1.4 The MessageBuilder Helper Class

You may notice that the Message interface defines retrieval methods for its payload and headers but no setters.
The reason for this is that a Message cannot be modified after its initial creation. Therefore, when a Message
instance is sent to multiple consumers (e.g. through a Publish Subscribe Channel), if one of those consumers
needs to send a reply with a different payload type, it will need to create a new Message. As a result, the
other consumers are not affected by those changes. Keep in mind, that multiple consumers may access the same
payload instance or header value, and whether such an instance is itself immutable is a decision left to the
developer. In other words, the contract for Messages is similar to that of an
unmodifiable Collection, and the MessageHeaders' map further exemplifies that; even though
the MessageHeaders class implements java.util.Map, any attempt to invoke a
put operation (or 'remove' or 'clear') on the MessageHeaders will result in an
UnsupportedOperationException.

Rather than requiring the creation and population of a Map to pass into the GenericMessage constructor, Spring
Integration does provide a far more convenient way to construct Messages: MessageBuilder.
The MessageBuilder provides two factory methods for creating Messages from either an existing Message or with a
payload Object. When building from an existing Message, the headers and payload of that
Message will be copied to the new Message:

Notice that the copyHeadersIfAbsent does not overwrite existing values. Also, in the
second example above, you can see how to set any user-defined header with setHeader.
Finally, there are set methods available for the predefined headers as well as a non-destructive method for
setting any header (MessageHeaders also defines constants for the pre-defined header names).

The priority header is only considered when using a PriorityChannel
(as described in the next chapter). It is defined as java.lang.Integer.

5. Message Routing

5.1 Router

5.1.1 Router Implementations

Since content-based routing often requires some domain-specific logic, most use-cases will require
Spring Integration's options for delegating to POJOs using the XML namespace support and/or Annotations.
Both of these are discussed below, but first we present a couple implementations that are available
out-of-the-box since they fulfill common requirements.

PayloadTypeRouter

A PayloadTypeRouter will send Messages to the channel as defined by payload-type
mappings.

Configuration of the PayloadTypeRouter is also supported via the namespace provided by Spring Integration (see Section B.2, “Namespace Support”),
which essentially simplifies configuration by combining the <router/> configuration and its corresponding implementation
defined using a <bean/> element
into a single and more concise configuration element.
The example below demonstrates a PayloadTypeRouter configuration which is equivalent to the one above using the namespace support:

HeaderValueRouter

A HeaderValueRouter will send Messages to the channel based on the individual header value mappings.
When a HeaderValueRouter is created it is initialized with the name of the header to be evaluated.
The value of the header could be one of two things:

1. Arbitrary value

2. Channel name

If arbitrary then additional mappings for these header values to channel names is required, otherwise no additional configuration is needed.

Spring Integration provides a simple namespace-based XML configuration to configure a HeaderValueRouter.
The example below demonstrates two types of namespace-based configuration for the HeaderValueRouter.

1. Configuration where mapping of header values to channels is required

During the resolution process this router may encounter channel resolution failures, causing an
exception. If you want to suppress such exceptions and send unresolved messages to the default output channel
(identified with the default-output-channel attribute) set ignore-channel-name-resolution-failures to true.
Normally, messages for which the header value is not explicitly mapped to a channel will be sent to the default-output-channel.
However, in cases where the header value is mapped to a channel name but the channel cannot be resolved, setting
the ignore-channel-name-resolution-failures
attribute to true will result in routing such messages to the default-output-channel.

2. Configuration where mapping of header values to channel names
is not required since header values themselves represent channel names

The two router implementations shown above share some common attributes, such as default-output-channel and resolution-required.
If resolution-required is set to true, and the router is unable to determine a target channel (e.g. there is
no matching payload for a PayloadTypeRouter and no default-output-channel has been specified), then an Exception
will be thrown.

RecipientListRouter

A RecipientListRouter will send each received Message to a statically defined
list of Message Channels:

Another convenient option when configuring a RecipientListRouter is to use Spring Expression Language (SpEL) support
as selectors for individual recipient channels. This is similar to using a Filter at the beginning of 'chain' to act as a "Selective Consumer".
However, in this case, it's all combined rather concisely into the router's configuration.

In the above configuration a SpEL expression identified by the selector-expression attribute will be evaluated to determine if this recipient
should be included in the recipient list for a given input Message. The evaluation result of the expression must be a boolean. If this
attribute is not defined, the channel will always be among the list of recipients.

5.1.2 Routing and Error handling

Spring Integration also provides a special type-based router called ErrorMessageExceptionTypeRouter for routing
Error Messages (Messages whose payload is a Throwable instance).
ErrorMessageExceptionTypeRouter is very similar to the PayloadTypeRouter.
In fact they are almost identical. The only difference is that while PayloadTypeRouter navigates
the instance hierarchy of a payload instance (e.g., payload.getClass().getSuperclass()) to find the most
specific type/channel mappings, the ErrorMessageExceptionTypeRouter navigates the hierarchy of
'exception causes' (e.g., payload.getCause()) to find the most specific Throwable type/channel mappings.

5.1.3 Configuring Router

Configuring a Content Based Router with XML

The "router" element provides a simple way to connect a router to an input channel and also accepts the
optional default-output-channel attribute. The ref attribute references the bean name of a custom Router implementation
(extending AbstractMessageRouter):

Alternatively, ref may point to a simple POJO that contains the @Router annotation (see below), or the
ref may be combined with an explicit method name. Specifying a method applies the same behavior
described in the @Router annotation section below.

<routerinput-channel="input"ref="somePojo"method="someMethod"/>

Using a ref attribute is generally recommended if the custom router implementation is referenced in other
<router> definitions. However if the custom router implementation should be scoped to a
single definition of the <router>, you may provide an inner bean definition:

Using both the ref attribute and an inner handler definition in the same <router> configuration
is not allowed, as it creates an ambiguous condition, and an Exception will be thrown.

Routers and the Spring Expression Language (SpEL)

Sometimes the routing logic may be simple and writing a separate class for it and configuring it as a bean may seem
like overkill. As of Spring Integration 2.0 we offer an alternative where you can now use
SpEL
to implement simple computations that previously required a custom POJO router. Generally a SpEL expression is evaluated and the
result is mapped to a channel:

In the above configuration the result channel will be computed by the SpEL expression which simply concatenates the value
of the payload with the literal String 'Channel'.

Another value of SpEL for configuring routers is that an expression can actually return a Collection,
effectively making every <router> a Recipient List Router. Whenever the expression returns
multiple channel values the Message will be forwarded to each channel.

Configuring a Router with Annotations

When using @Router to annotate a method, the method may return either a
MessageChannel or String type. In the latter case,
the endpoint will resolve the channel name as it does for the default output channel. Additionally, the method may return
either a single value or a collection. If a collection is returned, the reply message will be sent to multiple
channels. To summarize, the following method signatures are all valid.

In addition to payload-based routing, a Message may be routed based on metadata available within the
message header as either a property or attribute. In this case, a method annotated with @Router
may include a parameter annotated with @Header which is mapped to a header value as illustrated
below and documented in Section B.5, “Annotation Support”.

5.1.4 Dynamic Routers

So as you can see, Spring Integration provides quite a few different router configurations for common
content-based routing use cases as well as the option of implementing custom routers as POJOs.
For example PayloadTypeRouter provides a simple way to configure a router which computes channels
based on the payload type of the incoming Message while HeaderValueRouter provides the
same convenience in configuring a router which computes channels by evaluating the value
of a particular Message Header. There are also expression-based (SpEL) routers where the channel
is determined based on evaluating an expression. Thus, these type of routers exhibit some dynamic characteristics.

However these routers all require static configuration. Even in the case of
expression-based routers, the expression itself is defined as part of the router configuration which means that
the same expression operating on the same value will always result in the computation of the same channel.
This is acceptable in most cases since such routes are well defined and therefore predictable. But there are times when we
need to change router configurations dynamically so message flows may be routed to a different channel.

Example:

You might want to bring down some part of your system for maintenance and temporarily re-reroute
messages to a different message flow. Or you may want to introduce more granularity to your message flow by adding another
route to handle a more concrete type of java.lang.Number (in the case of PayloadTypeRouter).

Unfortunately with static router configuration to accomplish this you would have to bring down your entire application,
change the configuration of the router (change routes) and bring it back up. This is obviously not the solution.

The Dynamic Router
pattern describes the mechanisms by which one can change/configure routers dynamically without
bringing down the system or individual routers.

Before we get into the specifics of how this is accomplished in Spring Integration let's quickly summarize the
typical flow of the router, which consists of 3 simple steps:

Step 1 - Compute channel identifier which is a value calculated by the
router once it receives the Message. Typically it is a String or and instance of the actual
MessageChannel.

There is not much that can be done with regard to dynamic routing if Step 1 results in the actual instance of the
MessageChannel simply because the MessageChannel is the final product of any
router's job. However, if Step 1 results in a channel identifier that is not an instance of MessageChannel,
then there are quite a few possibilities to influence the process of deriving the Message Channel.
Lets look at couple of the examples in the context of the 3 steps mentioned above:

Within the context of the Payload Type Router the 3 steps mentioned above would be realized as:

Step 1 - Compute channel identifier which is the fully qualified name of the payload type
(e.g., java.lang.String).

Step 2 - Resolve channel identifier to channel name where
the result of the previous step is used to select the appropriate value from the payload type mapping
defined via mapping element.

Step 3 - Resolve channel name to the actual instance of the
MessageChannel where using ChannelResolver, the router will obtain a
reference to a bean (which is hopefully a MessageChannel) identified by the result of the
previous step.

In other words each step feeds the next step until the process completes.

Step 1 - Compute channel identifier which is the value of the header identified by the
header-name attribute.

Step 2 - Resolve channel identifier to channel name where
the result of the previous step is used to select the appropriate value from the general mapping
defined via mapping element.

Step 3 - Resolve channel name to the actual instance of the
MessageChannel where using ChannelResolver, the router will obtain a
reference to a bean (which is hopefully a MessageChannel) identified by the result of the
previous step.

The above two configurations of two different router types look almost identical.
However if we look at the alternate configuration of the HeaderValueRouter we clearly see that
there is no mapping sub element:

But the configuration is still perfectly valid. So the natural question is what about the mapping in the Step 2?

What this means is that Step 2 is now an optional step. If mapping is not defined then the channel identifier
value computed in Step 1 will automatically be treated as the channel name which will now be resolved to the
actual MessageChannel in the Step 3. What it also means is that Step 2 is one of the key steps to
provide dynamic characteristics to the routers, since it introduces a process which
allows you to change the way 'channel identifier' resolves to 'channel name',
thus influencing the process of determining the final instance of the MessageChannel from the initial
channel identifier.

For Example:

In the above configuration let's assume that the testHeader value is 'kermit' which is now a channel identifier
(Step 1). Since there is no mapping in this router, resolving this channel identifier to a channel name
(Step 2) is impossible and this channel identifier is now treated as channel name. However what if
there was a mapping but for a different value? The end result would still be the same and that is:
if new value cannot be determined through the process of resolving the 'channel identifier' to a 'channel name',
such 'channel identifier' becomes 'channel name'.

So all that is left is for Step 3 to resolve the channel name ('kermit') to an actual instance of the
MessageChannel identified by this name. That will be done via the default
ChannelResolver implementation which is a BeanFactoryChannelResolver. It
basically does a bean lookup for the name provided. So now all messages which contain the header/value pair as testHeader=kermit
are going to be routed to a MessageChannel whose bean name (id) is 'kermit'.

But what if you want to route these messages to the 'simpson' channel? Obviously changing a static configuration will work,
but will also require bringing your system down. However if you had access to the channel identifier map, then you
could just introduce a new mapping where the header/value pair is now kermit=simpson, thus allowing Step 2 to treat
'kermit' as a channel identifier while resolving it to 'simpson' as the channel name .

The same obviously applies for PayloadTypeRouter where you can now remap or remove a particular payload type
mapping. In fact, it applies to every other router including expression-based routers since their computed values
will now have a chance to go through Step 2 to be additionally resolved to the actual channel name.

In Spring Integration 2.0 the routers hierarchy underwent significant refactoring so that now any router that is a subclass of the
AbstractMessageRouter (which includes all framework defined routers) is a Dynamic Router simply because the
channelIdentiferMap is defined at the AbstractMessageRouter level. That map's setter method is
exposed as a public method along with 'setChannelMapping' and 'removeChannelMapping' methods. These allow you to change/add/remove
router mappings at runtime as long as you have a reference to the router itself. It also means that you could expose these same
configuration options via JMX (see Section 8.1, “JMX Support”) or the Spring Integration ControlBus (see Section 8.3, “Control Bus”) functionality.

Control Bus

One way to manage the router mappings is through the Control Bus
pattern which exposes a Control Channel where you can send
control messages to manage and monitor Spring Integration components, including routers.
For more information about the Control Bus see Section 8.3, “Control Bus”. Typically you would send a control message asking to invoke a
particular operation on a particular managed component (e.g., router). The two managed operations (methods) that are
specific to changing the router resolution process are:

public void setChannelMapping(String channelIdentifier, String channelName) -
will allow you to add a new or modify an existing mapping between channel identifier and channel name

public void removeChannelMapping(String channelIdentifier) -
will allow you to remove a particular channel mapping, thus disconnecting the relationship between
channel identifier and channel name

You can also expose a router instance with Spring's JMS support and then use your favorite JMX client (e.g., JConsole) to
manage those operations (methods) for changing the router's configuration. For more information on Spring Integration
management and monitoring please visit Section 8.1, “JMX Support”.

5.2 Filter

5.2.1 Introduction

Message Filters are used to decide whether a Message should be passed along or dropped based on some criteria
such as a Message Header value or Message content itself. Therefore, a Message Filter is similar
to a router, except that for each Message received from the filter's input channel, that same Message may or may
not be sent to the filter's output channel. Unlike the router, it makes no decision regarding
which Message Channel to send the Message to but only decides whether to send.

Note

As you will see momentarily, the Filter also supports a discard channel, so in certain cases it
can play the role of a very simple router (or "switch") based on a boolean condition.

In Spring Integration, a Message Filter may be configured as a Message Endpoint that delegates to an
implementation of the MessageSelector interface. That interface is itself quite
simple:

In combination with the namespace and SpEL, very powerful filters can be configured with very little java code.

5.2.2 Configuring Filter

The <filter> element is used to create a Message-selecting endpoint. In addition to "input-channel
and output-channel attributes, it requires a ref. The ref may point to a
MessageSelector implementation:

Alternatively, the method attribute can be added at which point the ref may refer to any object.
The referenced method may expect either the Message type or the payload type of
inbound Messages. The method must return a boolean value. If the method returns 'true',
the Message will be sent to the output-channel.

If the selector or adapted POJO method returns false, there are a few settings that control the
handling of the rejected Message. By default (if configured like the example above), rejected Messages will
be silently dropped. If rejection should instead result in an error condition, then set the
throw-exception-on-rejection attribute to true:

Message Filters are commonly used in conjunction with a Publish Subscribe Channel. Many filter endpoints may
be subscribed to the same channel, and they decide whether or not to pass the Message to the next endpoint which
could be any of the supported types (e.g. Service Activator). This provides a reactive
alternative to the more proactive approach of using a Message Router with a single
Point-to-Point input channel and multiple output channels.

Using a ref attribute is generally recommended if the custom filter implementation is referenced in other
<filter> definitions. However if the custom filter implementation is scoped to a
single <filter> element, provide an inner bean definition:

Using both the ref attribute and an inner handler definition in the same <filter> configuration
is not allowed, as it creates an ambiguous condition, and an Exception will be thrown.

With the introduction of SpEL support, Spring Integration added the expression attribute to the filter
element. It can be used to avoid Java entirely for simple filters.

<filterinput-channel="input"expression="payload.equals('nonsense')"/>

The string passed as the expression attribute will be evaluated as a SpEL expression with the Message available in
the evaluation context.
If it is necessary to include the result of an expression in the scope of the application context you can use the
#{} notation as defined in the
SpEL reference documentation
.

If the Expression itself needs to be dynamic, then an 'expression' sub-element may be used. That provides a level of
indirection for resolving the Expression by its key from an ExpressionSource. That is a strategy interface that you
can implement directly, or you can rely upon a version available in Spring Integration that loads Expressions from
a "resource bundle" and can check for modifications after a given number of seconds. All of this is demonstrated in
the following configuration sample where the Expression could be reloaded within one minute if the underlying file
had been modified. If the ExpressionSource bean is named "expressionSource", then it is not necessary to provide the
source attribute on the <expression> element, but in this case it's shown for completeness.

Then, the 'config/integration/expressions.properties' file (or any more specific version with a locale extension
to be resolved in the typical way that resource-bundles are loaded) would contain a key/value pair:

filterPatterns.example=payload > 100

Note

All of these examples that use expression as an attribute or sub-element can also be applied within
transformer, router, splitter, service-activator, and header-enricher elements. Of course, the semantics/role
of the given component type would affect the interpretation of the evaluation result in the same way that the
return value of a method-invocation would be interpreted. For example, an expression can return Strings that are
to be treated as Message Channel names by a router component. However, the underlying functionality of evaluating
the expression against the Message as the root object, and resolving bean names if prefixed with '@' is consistent
across all of the core EIP components within Spring Integration.

5.3 Splitter

5.3.1 Introduction

The Splitter is a component whose role is to partition a message in
several parts, and send the resulting messages to be processed
independently. Very often, they are upstream producers in a pipeline that
includes an Aggregator.

5.3.2 Programming model

The API for performing splitting consists of one base class,
AbstractMessageSplitter, which is a
MessageHandler implementation,
encapsulating features which are common to splitters, such as filling in
the appropriate message headers CORRELATION_ID, SEQUENCE_SIZE, and
SEQUENCE_NUMBER on the messages that are produced. This enables tracking
down the messages and the results of their processing (in a typical
scenario, these headers would be copied over to the messages that are
produced by the various transforming endpoints), and use them, for
example, in a
Composed Message Processor scenario.

To implement a specific Splitter in an application,
extend AbstractMessageSplitter and implement the splitMessage method,
which contains logic for splitting the messages. The return
value may be one of the following:

a Collection (or subclass thereof) or an array of
Message objects -
in this case the messages will be sent as such (after the
CORRELATION_ID, SEQUENCE_SIZE and SEQUENCE_NUMBER are populated).
Using this approach gives more control to the developer, for example
for populating custom message headers as part of the splitting
process.

a Collection (or subclass thereof) or an array of
non-Message objects - works like the prior case, except that each collection
element will be used as a Message payload. Using this approach allows
developers to focus on the domain objects without having to consider
the Messaging system and produces code that is easier to test.

a Message or non-Message object
(but not a Collection or an Array) - it works like the previous cases,
except a single message will be sent out.

In Spring Integration, any POJO can implement the splitting
algorithm, provided that it defines a method that accepts a single
argument and has a return value. In this case, the return value of the
method will be interpreted as described above. The input argument might
either be a Message or a simple POJO.
In the latter case, the splitter will receive the payload of the incoming message.
Since this decouples the code from the Spring Integration API and will typically be easier
to test, it is the recommended approach.

A reference to a bean defined in the application context. The
bean must implement the splitting logic as described in the section
above .Optional.
If reference to a bean is not provided, then it is assumed that the payload
of the Message that arrived on the input-channel is
an implementation of java.util.Collection
and the default splitting logic will be applied to the Collection,
incorporating each individual element into a Message and sending it to the output-channel.

The method (defined on the bean specified above) that
implements the splitting logic.
Optional.

The input channel of the splitter.
Required.

The channel to which the splitter will send the results of
splitting the incoming message. Optional (because incoming
messages can specify a reply channel themselves).

Using a ref attribute is generally recommended if the custom splitter implementation may be referenced in other
<splitter> definitions. However if the custom splitter handler implementation should be scoped to a
single definition of the <splitter>, configure an inner bean definition:

Using both a ref attribute and an inner handler definition in the same <splitter>
configuration is not allowed, as it creates an ambiguous condition and will result in an Exception being thrown.

Configuring a Splitter with Annotations

The @Splitter annotation is
applicable to methods that expect either the
Message type or the message payload type,
and the return values of the method should be a Collection of any type.
If the returned values are not actual Message
objects, then each item will be wrapped in a Message as its payload. Each
message will be sent to the designated output channel for the endpoint
on which the @Splitter is defined.

5.4 Aggregator

5.4.1 Introduction

Basically a mirror-image of the Splitter, the Aggregator is a type
of Message Handler that receives multiple Messages and combines them into
a single Message. In fact, an Aggregator is often a downstream consumer in a
pipeline that includes a Splitter.

Technically, the Aggregator is more complex than a Splitter, because
it is stateful as it must hold the Messages to be aggregated and determine
when the complete group of Messages is ready to be aggregated. In order to do
this it requires a MessageStore.

5.4.2 Functionality

The Aggregator combines a group of related messages, by correlating
and storing them, until the group is deemed complete. At that point, the
Aggregator will create a single message by processing the whole group, and
will send the aggregated message as output.

Implementing an Aggregator requires providing the logic
to perform the aggregation (i.e., the creation of a single message
from many). Two related concepts are correlation and
release.

Correlation determines how messages are grouped for aggregation.
In Spring Integration correlation is done by default based on the CORRELATION_ID message
header. Messages with the same CORRELATION_ID will be grouped
together. However, the correlation strategy may be customized to allow
other ways of specifying how the messages should be grouped together by
implementing a CorrelationStrategy (see below).

To determine the point at which a group of messages is ready to be processed, a
ReleaseStrategy is consulted.
The default release strategy for the Aggregator will release a group when all
messages included in a sequence are present, based on the SEQUENCE_SIZE header.
This default strategy may be overridden by providing a reference to a
custom ReleaseStrategy implementation.

5.4.3 Programming model

The Aggregation API consists of a number of classes:

The interface MessageGroupProcessor, and
its subclasses:
MethodInvokingAggregatingMessageGroupProcessor and
ExpressionEvaluatingMessageGroupProcessor

The ReleaseStrategy interface and its default
implementation SequenceSizeReleaseStrategy

The CorrelationStrategy interface and its default
implementation HeaderAttributeCorrelationStrategy

CorrelatingMessageHandler

The CorrelatingMessageHandler is a
MessageHandler implementation, encapsulating the common
functionalities of an Aggregator (and other correlating use cases),
which are:

correlating messages into a group to be aggregated

maintaining those messages in a MessageStore until the group
can be released

deciding when the group can be released

aggregating the released group into a single message

recognizing and responding to an expired group

The responsibility of deciding how the messages should
be grouped together is delegated to a CorrelationStrategy
instance. The responsibility of deciding whether the message group can
be released is delegated to a ReleaseStrategy
instance.

Here is a brief highlight of the base
AbstractAggregatingMessageGroupProcessor (the
responsibility of implementing the aggregatePayloads method is left to
the developer):

As for actual processing of the message group, the default implementation is the
DefaultAggregatingMessageGroupProcessor. It creates a
single Message whose payload is a List of the payloads received for a
given group. This works well for simple Scatter Gather implementations with either a Splitter, Publish
Subscribe Channel, or Recipient List Router upstream.

Note

When using a Publish Subscribe Channel or Recipient List Router
in this type of scenario, be sure to enable the flag to
apply-sequence. That will add the necessary
headers (CORRELATION_ID, SEQUENCE_NUMBER and SEQUENCE_SIZE). That
behavior is enabled by default for Splitters in Spring Integration,
but it is not enabled for the Publish Subscribe Channel or Recipient
List Router because those components may be used in a variety of
contexts in which these headers are not necessary.

When implementing a specific aggregator strategy for an application,
a developer can extend
AbstractAggregatingMessageGroupProcessor and implement the
aggregatePayloads method. However, there are better solutions, less
coupled to the API, for implementing the aggregation logic which can be configured easily
either through XML or through annotations.

In general, any POJO can implement the
aggregation algorithm if it provides a method that
accepts a single java.util.List as an argument
(parameterized lists are supported as well). This method will be invoked for aggregating
messages as follows:

if the argument is a java.util.List<T>, and the
parameter type T is assignable to Message, then the whole list of
messages accumulated for aggregation will be sent to the aggregator

if the argument is a non-parameterized java.util.List or the
parameter type is not assignable to Message, then the method will
receive the payloads of the accumulated messages

if the return type is not assignable to Message, then it will
be treated as the payload for a Message that will be created
automatically by the framework.

Note

In the interest of code simplicity, and promoting best practices
such as low coupling, testability, etc., the preferred way of
implementing the aggregation logic is through a POJO, and using the
XML or annotation support for configuring it in the application.

ReleaseStrategy

In general, any POJO can implement the
completion decision logic if it provides a method that accepts a single
java.util.List as an argument (parameterized lists
are supported as well), and returns a boolean value. This method will be
invoked after the arrival of each new message, to decide whether the group
is complete or not, as follows:

if the argument is a java.util.List<T>, and the
parameter type T is assignable to Message, then the whole list of
messages accumulated in the group will be sent to the method

if the argument is a non-parametrized java.util.List or the
parameter type is not assignable to Message, then the method will
receive the payloads of the accumulated messages

the method must return true if the message group is ready for
aggregation, and false otherwise.

As you can see based on the above signatures, the POJO-based Release Strategy will be passed a Collection of unmarked Messages
if you need access to the whole Message or Collection of payload objects if the type parameter is
anything other than Message. Typically this would satisfy the majority of use cases. However if
for some reason you need to access the full MessageGroup - which contains unmarked and marked Messages -
then you should simply provide an implementation of the ReleaseStrategy interface.

When the group is released for aggregation, all its unmarked
messages are processed and then marked so they will not be processed
again. If the group is also complete (i.e. if all messages from a
sequence have arrived or if there is no sequence defined), then the group
is removed from the message store. Partial sequences can be released, in
which case the next time the ReleaseStrategy is called it
will be presented with a group containing marked messages (already
processed) and unmarked messages (potentially a new partial
sequence).

Spring Integration provides an out-of-the box implementation for
ReleaseStrategy, the
SequenceSizeReleaseStrategy. This implementation consults the
SEQUENCE_NUMBER and SEQUENCE_SIZE headers of each arriving message to decide
when a message group is complete and ready to be aggregated. As shown
above, it is also the default strategy.

CorrelationStrategy

The method returns an Object which represents the correlation key
used for associating the message with a message group. The key must satisfy the
criteria used for a key in a Map with respect to the implementation of
equals() and hashCode().

In general, any POJO can implement the
correlation logic, and the rules for mapping a message to a
method's argument (or arguments) are the same as for a
ServiceActivator (including support for @Header
annotations). The method must return a value, and the value must not be
null.

Spring Integration provides an out-of-the box implementation for
CorrelationStrategy, the
HeaderAttributeCorrelationStrategy. This implementation
returns the value of one of the message headers (whose name is specified
by a constructor argument) as the correlation key. By default, the
correlation strategy is a HeaderAttributeCorrelationStrategy returning
the value of the CORRELATION_ID header attribute. If you have a custom header name
you would like to use for correlation, then simply configure that on an instance of
HeaderAttributeCorrelationStrategy and provide that as a
reference for the Aggregator's correlation-strategy.

5.4.4 Configuring Aggregator

Configuring an Aggregator with XML

Spring Integration supports the configuration of an aggregator via
XML through the <aggregator/> element. Below you can see an example
of an aggregator.

Lifecycle attribute signaling if aggregator should be started during Application Context startup.
Optional (default is 'true').

The channel from which where aggregator will receive messages.
Required.

The channel to which the aggregator will send the aggregation
results. Optional (because incoming messages can specify a
reply channel themselves via 'replyChannel' Message Header).

The channel to which the aggregator will send the messages that
timed out (if send-partial-result-on-expiry is
false). Optional.

A reference to a MessageGroupStore used
to store groups of messages under their correlation key until they are
complete. Optional, by default a volatile
in-memory store.

Order of this aggregator when more than one handle is subscribed to the same DirectChannel
(use for load balancing purposes).
Optional.

Indicates if partially aggregated messages should be released when their storage time has expired
(see MessageGroupStore.expireMessageGroups(long)).
Optional.

The timeout interval for sending the aggregated messages to the output or
reply channel. Optional.

A reference to a bean that implements the message correlation (grouping)
algorithm. The bean can be an implementation of the CorrelationStrategy
interface or a POJO. In the latter case the correlation-strategy-method attribute must be defined
as well. Optional (by default, the aggregator will use
the CORRELATION_ID header) .

A method defined on the bean referenced by
correlation-strategy, that implements the
correlation decision algorithm. Optional, with
restrictions (requires correlation-strategy to be
present).

A reference to a bean defined in the application context. The bean must implement the aggregation logic
as described above. Optional (by default the list of aggregated Messages will become a
payload of the output message).

A method defined on the bean referenced by ref,
that implements the message aggregation
algorithm. Optional, depends on ref attribute being defined.

A reference to a bean that implements the release strategy.
The bean can be an implementation of the ReleaseStrategy interface
or a POJO. In the latter case the release-strategy-method
attribute must be defined as well. Optional (by default, the
aggregator will use the SEQUENCE_SIZE header attribute).

A method defined on the bean referenced by
release-strategy, that implements the
completion decision algorithm. Optional, with
restrictions (requires release-strategy to be
present).

Using a ref attribute is generally recommended if a custom
aggregator handler implementation may be referenced in other
<aggregator> definitions. However if a custom
aggregator implementation is only being used by a single
definition of the <aggregator>, you can use an inner
bean definition (starting with version 1.0.3) to configure the aggregation
POJO within the <aggregator> element:

For example, this aggregator would group numbers by some criterion
(in our case the remainder after dividing by 10) and will hold the group
until the sum of the numbers provided by the payloads exceeds a
certain value.

Note

Wherever it makes sense, the release strategy method, correlation
strategy method and the aggregator method can be combined in a single
bean (all of them or any two).

Aggregators and Spring Expression Language (SpEL)

Since Spring Integration 2.0, the various strategies (correlation, release, and aggregation) may be handled with
SpEL
which is recommended if the logic behind such release strategy is relatively simple.
Let's say you have a legacy component that was designed to receive an array of objects. We know that the default release
strategy will assemble all aggregated messages in the List. So now we have two problems. First we need to extract
individual messages from the list, and then we need to extract the payload of each message and assemble
the array of objects (see code below).

In the above configuration we are using a Collection Projection expression
to assemble a new collection from the payloads of all messages in the list and then transforming it to an Array, thus
achieving the same result as the java code above.

The same expression-based approach can be applied when dealing with custom Release and
Correlation strategies.

Instead of defining a bean for a custom CorrelationStrategy via
the correlation-strategy attribute, you can implement your simple correlation logic
via a SpEL expression and configure it via the correlation-strategy-expression attribute.

For example:

correlation-strategy-expression="payload.person.id"

In the above example it is assumed that the payload has an attribute person with an id
which is going to be used to correlate messages.

Likewise, for the ReleaseStrategy you can implement your release logic as
a SpEL expression and configure it via the release-strategy-expression attribute.
The only difference is that since ReleaseStrategy is passed the List of Messages, the root object in the SpEL
evaluation context is the List itself. That List can be referenced as #this within the expression.

For example:

release-strategy-expression="#this.size() gt 5"

In this example the root object of the SpEL Evaluation Context is the
MessageGroup itself, and you are simply stating
that as soon as there are more than 5 messages in this group, it should be released.

An annotation indicating that this method shall be used as an
aggregator. Must be specified if this class will be used as an
aggregator.

An annotation indicating that this method shall be
used as the release strategy of an aggregator. If not present on any
method, the aggregator will use the
SequenceSizeReleaseStrategy.

An annotation indicating that this method shall be
used as the correlation strategy of an aggregator. If no correlation
strategy is indicated, the aggregator will use the
HeaderAttributeCorrelationStrategy based on CORRELATION_ID.

All of the configuration options provided by the xml element are
also available for the @Aggregator annotation.

The aggregator can be either referenced explicitly from XML or, if
the @MessageEndpoint is defined on the class, detected automatically
through classpath scanning.

5.4.5 Managing State in an Aggregator:
MessageGroupStore

Aggregator (and some other patterns in Spring Integration) is a
stateful pattern that requires decisions to be made based on a group of
messages that have arrived over a period of time, all with the same
correlation key. The design of the interfaces in the stateful patterns
(e.g. ReleaseStrategy) is driven by the principle
that the components (whether defined by the framework or a user) should be able to remain stateless.
All state is carried by the MessageGroup and its
management is delegated to the
MessageGroupStore.

The MessageGroupStore accumulates state
information in MessageGroups while waiting for
a release strategy to be triggered, and that event might not ever happen.
So to prevent stale messages from lingering, and for volatile stores to
provide a hook for cleaning up when the application shuts down, the
MessageGroupStore allows the user to register
callbacks to apply to its MessageGroups when they
expire. The interface is very straightforward:

The callback has direct access to the store and the message group
so it can manage the persistent state (e.g. by removing the group from the
store entirely).

The MessageGroupStore maintains a list of these callbacks
which it applies, on demand, to all messages whose timestamp is earlier than a time
supplied as a parameter (see the registerMessageGroupExpiryCallback(..)
and expireMessageGroups(..) methods above).

The expireMessageGroups method can be called with a timeout value.
Any message older than the current time minus this value will be expired,
and have the callbacks applied. Thus it is the user of the store that
defines what is meant by message group "expiry".

As a convenience for users, Spring Integration provides a wrapper
for the message expiry in the form of a
MessageGroupStoreReaper:

The reaper is a Runnable, and all that is happening
in the example above is that the message group store's expire method is being called
once every 10 seconds. The timeout itself is 30 seconds.

In addition to the reaper, the expiry callbacks are invoked when the application
shuts down via a lifecycle callback in the CorrelatingMessageHandler.

The CorrelatingMessageHandler registers its
own expiry callback, and this is the link with the boolean flag
send-partial-result-on-expiry in the XML configuration of the
aggregator. If the flag is set to true, then when the expiry callback is
invoked, any unmarked messages in groups that are not yet released can
be sent on to the output channel.

5.5 Resequencer

5.5.1 Introduction

Related to the Aggregator, albeit different from a functional
standpoint, is the Resequencer.

5.5.2 Functionality

The Resequencer works in a similar way to the Aggregator, in the
sense that it uses the CORRELATION_ID to store messages in groups, the
difference being that the Resequencer does not process the messages in any
way. It simply releases them in the order of their SEQUENCE_NUMBER header
values.

With respect to that, the user might opt to release all messages at
once (after the whole sequence, according to the SEQUENCE_SIZE, has been
released), or as soon as a valid sequence is available.

5.5.3 Configuring a Resequencer

Configuring a resequencer requires only including the appropriate
element in XML.

The channel to which the resequencer will send the reordered
messages. Optional.

The channel to which the resequencer will send the messages that
timed out (if send-partial-result-on-timeout is
false). Optional.

Whether to send out ordered sequences as soon as they are
available, or only after the whole message group arrives.
Optional (false by default).

If this flag is not specified (so a complete sequence is defined by the sequence headers) then it may make sense to provide a custom
Comparator
to be used to order the messages when sending (use the XML attribute
comparator
to point to a bean definition). If
release-partial-sequences
is true then there is no way with a custom comparator to define a partial sequence. To do that you would have to provide a
release-strategy
(also a reference to another bean definition, either a POJO or a
ReleaseStrategy
).

A reference to a MessageGroupStore that can be
used to store groups of messages under their correlation key until
they are complete. Optional with default a
volatile in-memory store.

Since there is no custom behavior to be implemented in Java classes for resequencers, there is no annotation support for it.

5.6 Message Handler Chain

5.6.1 Introduction

The MessageHandlerChain is an implementation of
MessageHandler that can be configured as a single Message Endpoint while
actually delegating to a chain of other handlers, such as Filters, Transformers, Splitters, and so on.
This can lead to a much simpler configuration when several handlers need to be connected in a fixed, linear
progression. For example, it is fairly common to provide a Transformer before other components. Similarly, when
providing a Filter before some other component in a chain, you are essentially creating a
Selective Consumer. In either case, the
chain only requires a single input-channel and a single output-channel eliminating
the need to define channels for each individual component.

Tip

Spring Integration's Filter provides a boolean property throwExceptionOnRejection.
When providing multiple Selective Consumers on the same point-to-point channel with different acceptance criteria,
this value should be set to 'true' (the default is false) so that the dispatcher will know that the Message was
rejected and as a result will attempt to pass the Message on to other subscribers. If the Exception were not
thrown, then it would appear to the dispatcher as if the Message had been passed on successfully even though
the Filter had dropped the Message to prevent further processing. If you do indeed want
to "drop" the Messages, then the Filter's 'discard-channel' might be useful since it does give you a chance
to perform some operation with the dropped message (e.g. send to a JMS queue or simply write to a log).

The handler chain simplifies configuration while internally maintaining the same degree of loose coupling between
components, and it is trivial to modify the configuration if at some point a non-linear arrangement is required.

Internally, the chain will be expanded into a linear setup of the listed endpoints, separated by anonymous channels.
The reply channel header will not be taken into account within the chain: only after the last handler is invoked
will the resulting message be forwarded on to the reply channel or the chain's output channel. Because of this
setup all handlers except the last required to implement the MessageProducer interface (which provides a
'setOutputChannel()' method). The last handler only needs an output channel if the outputChannel on the
MessageHandlerChain is set.

Note

As with other endpoints, the output-channel is optional. If there is a reply Message at the end of the
chain, the output-channel takes precedence, but if not available, the chain handler will check for a
reply channel header on the inbound Message as a fallback.

In most cases there is no need to implement MessageHandlers yourself. The next section will focus on namespace
support for the chain element. Most Spring Integration endpoints, like Service Activators and Transformers, are
suitable for use within a MessageHandlerChain.

5.6.2 Configuring Chain

The <chain> element provides an input-channel attribute, and if the last element in the chain is capable
of producing reply messages (optional), it also supports an output-channel attribute. The sub-elements are then
filters, transformers, splitters, and service-activators. The last element may also be a router.

The <header-enricher> element used in the above example will set a message header named "foo" with a value
of "bar" on the message. A header enricher is a specialization of Transformer
that touches only header values. You could obtain the same result by implementing a MessageHandler that did the
header modifications and wiring that as a bean, but the header-enricher is obviously a simpler option.

Sometimes you need to make a nested call to another chain from within a chain and then come
back and continue execution within the original chain.
To accomplish this you can utilize a Messaging Gateway by including a <gateway> element.
For example:

In the above example the nested-chain-a will be called at the end of
main-chain processing by the 'gateway' element configured there. While in
nested-chain-a a call to a nested-chain-b will be made
after header enrichment and then it will come back to finish execution in nested-chain-b.
Finally the flow returns to the main-chain. When the nested version of a <gateway>
element is defined in the chain, it does not require the service-interface attribute.
Instead, it simple takes the message in its current state and places it on the channel defined via
the request-channel attribute. When the downstream flow initiated by that gateway completes,
a Message will be returned to the gateway and continue its journey within
the current chain.

6. Message Transformation

6.1 Transformer

6.1.1 Introduction

Message Transformers play a very important role in enabling the loose-coupling of Message Producers and Message
Consumers. Rather than requiring every Message-producing component to know what type is expected by the next
consumer, Transformers can be added between those components. Generic transformers, such as one that converts a
String to an XML Document, are also highly reusable.

For some systems, it may be best to provide a
Canonical Data Model, but Spring
Integration's general philosophy is not to require any particular format. Rather, for maximum flexibility, Spring
Integration aims to provide the simplest possible model for extension. As with the other endpoint types, the use
of declarative configuration in XML and/or Annotations enables simple POJOs to be adapted for the role of Message
Transformers. These configuration options will be described below.

Note

For the same reason of maximizing flexibility, Spring does not require XML-based Message payloads.
Nevertheless, the framework does provide some convenient Transformers for dealing with XML-based payloads if
that is indeed the right choice for your application. For more information on those transformers, see
Chapter 24, XML Support - Dealing with XML Payloads.

6.1.2 Configuring Transformer

Configuring Transformer with XML

The <transformer> element is used to create a Message-transforming endpoint. In addition to "input-channel"
and "output-channel" attributes, it requires a "ref". The "ref" may either point to an Object that contains the
@Transformer annotation on a single method (see below) or it may be combined with an explicit method name value
provided via the "method" attribute.

Using a "ref" attribute is generally recommended if the custom transformer handler implementation can be reused in
other <transformer> definitions. However if the custom transformer handler implementation should
be scoped to a single definition of the <transformer>, you can define an inner bean definition:

Using both the "ref" attribute and an inner handler definition in the same <transformer>
configuration is not allowed, as it creates an ambiguous condition and will result in an Exception being thrown.

The method that is used for transformation may expect either the Message type or
the payload type of inbound Messages. It may also accept Message header values either individually or as a full
map by using the @Header and @Headers parameter annotations respectively. The return value of the method can be
any type. If the return value is itself a Message, that will be passed along to
the transformer's output channel.

As of Spring Integration 2.0, a Message Transformer's transformation method can no longer return null.
Returning null will result in an exception since a Message Transformer should always be expected to
transform each source Message into a valid target Message. In other words, a Message Transformer should not be used
as a Message Filter since there is a dedicated <filter> option for that. However, if you do need this type of
behavior (where a component might return NULL and that should not be considered an error), a
service-activator could be used. Its requires-reply value is FALSE by default,
but that can be set to TRUE in order to have Exceptions thrown for NULL return values as with the transformer.

Transformers and Spring Expression Language (SpEL)

Just like Routers, Aggregators and other components, as of Spring Integration 2.0 Transformers can also benefit from SpEL support
(http://static.springsource.org/spring/docs/3.0.x/spring-framework-reference/html/expressions.html)
whenever transformation logic is relatively simple.

In the above configuration we are achieving a simple transformation of the payload with a simple SpEL
expression and without writing a custom transformer. Our payload (assuming String) will be
upper-cased and concatenated with the current timestamp with some simple formatting.

Common Transformers

There are also a few Transformer implementations available out of the box. Because, it is fairly common
to use the toString() representation of an Object, Spring Integration provides an
ObjectToStringTransformer whose output is a Message with a String payload. That String
is the result of invoking the toString() operation on the inbound Message's payload.

<object-to-string-transformerinput-channel="in"output-channel="out"/>

A potential example for this would be sending some arbitrary object to the 'outbound-channel-adapter' in the
file namespace. Whereas that Channel Adapter only supports String, byte-array, or
java.io.File payloads by default, adding this transformer immediately before the
adapter will handle the necessary conversion. Of course, that works fine as long as the result of the
toString() call is what you want to be written to the File. Otherwise, you can
just provide a custom POJO-based Transformer via the generic 'transformer' element shown previously.

Tip

When debugging, this transformer is not typically necessary since the 'logging-channel-adapter' is capable
of logging the Message payload. Refer to the section called “Wire Tap” for more detail.

If you need to serialize an Object to a byte array or deserialize a byte array back into an Object,
Spring Integration provides symmetrical serialization transformers. These will use standard Java serialization
by default, but you can provide an implementation of Spring 3.0's Serializer or Deserializer strategies via the
'serializer' and 'deserializer' attributes, respectively.

Spring Integration also provides Object-to-Map and Map-to-Object transformers which
utilize the Spring Expression Language (SpEL) to serialize and de-serialize the object graphs. The object hierarchy is introspected
to the most primitive types (String, int, etc.). The path to this type is described via SpEL, which becomes the key in the
transformed Map. The primitive type becomes the value.

... will be transformed to a Map which looks like this:
{person.name=George, person.child.name=Jenna, person.child.nickNames[0]=Bimbo . . . etc}

The SpEL-based Map allows you to describe the object structure without sharing the actual types allowing
you to restore/rebuild the object graph into a differently typed Object graph as long as you maintain the structure.

For example:
The above structure could be easily restored back to the following Object graph via the Map-to-Object transformer:

NOTE: 'ref' and 'type' attributes are mutually exclusive. You can only use one.
Also, if using the 'ref' attribute, you must point to a 'prototype' scoped bean, otherwise
a BeanCreationException will be thrown.

Configuring a Transformer with Annotations

The @Transformer annotation can also be added to methods that expect either the
Message type or the message payload type. The return value will be handled in the
exact same way as described above in the section describing the <transformer> element.

6.1.3 Header Filter

Some times your transformation use case might be as simple as removing a few headers.
For such a use case, Spring Integration provides a Header Filter which allows you to specify certain header names
that should be removed from the output Message (e.g. for security reasons or a value that was only needed temporarily).
Basically the Header Filter is the opposite of the Header Enricher.
The latter is discussed in Section 6.2.2, “Header Enricher”

As you can see, configuration of a Header Filter is quite simple. It is a typical endpoint with input/output channels
and a header-names attribute. That attribute accepts the names of the header(s) (delimited by commas if there are multiple)
that need to be removed. So, in the above example the headers named 'lastName' and 'state' will not be present on the outbound Message.

6.2 Content Enricher

6.2.1 Introduction

At times you may have a requirement to enhance a request with more information than was
provided by the target system. The Content Enricher pattern
describes various scenarios as well as the component (Enricher), which allows you to address such requirements.

6.2.2 Header Enricher

If you only need to add headers to a Message, and they are not dynamically determined by the Message content,
then referencing a custom implementation of a Transformer may be overkill. For that reason,
Spring Integration provides support for the Header Enricher pattern. It is exposed via
the <header-enricher> element.

In the above configuration you can clearly see that for well-known headers such as errorChannel,
correlationId, priority, replyChanneletc., instead of using generic
<header> sub-elements where you would have to provide both header 'name' and 'value',
you can use convenient sub-elements to set those values directly.

POJO Support

Often a header value cannot be defined statically and has to be determined dynamically based on some content in the Message. That is why
Header Enricher allows you to also specify a bean 'ref' and 'method' that will calculate the
header value. Let's look at the following configuration:

In Spring Integration 2.0 we have introduced the convenience of the
Spring Expression Language (SpEL)
to help configure many different components. The Header Enricher is one of them.
Looking again at the POJO example above, you can see that the computation logic to determine the header value is actually pretty simple.
A natural question would be: "is there a simpler way to accomplish this?". That is where SpEL shows its true power.

As you can see, by using SpEL for such simple cases, we no longer have to provide a separate class and configure
it in the application context. All we need is the expression attribute configured with a valid
SpEL expression. The 'payload' and 'headers' variables are bound to the SpEL Evaluation Context,
giving you full access to the incoming Message.

Adapter specific Header Enrichers

As you go through the manual, you will see that as an added convenience,
Spring Integration also provides adapter specific Header Enrichers (e.g., MAIL, XMPP, etc.)

6.3 Claim Check

6.3.1 Introduction

In the earlier sections we've covered several Content Enricher type components that help you deal with situations where a
message is missing a piece of data. We also discussed Content Filtering which lets you remove data items from a message.
However there are times when we want to hide data temporarily. For example, in a distributed system we may receive a
Message with a very large payload. Some intermittent message processing steps may not need access to this payload and some may only
need to access certain headers, so carrying the large Message payload through each processing step may cause performance degradation,
may produce a security risk, and may make debugging more difficult.

The Claim Check pattern describes a mechanism that allows you
to store data in a well known place while only maintaining a pointer (Claim Check) to where that data is located. You can pass that
pointer around as a payload of a new Message thereby allowing any component within the message flow to get the actual data as soon as
it needs it. This approach is very similar to the Certified Mail process where you'll get a Claim Check in your mailbox and
would have to go to the Post Office to claim your actual package. Of course it's also the same idea as baggage-claim on a flight
or in a hotel.

In the above configuration the Message that is received on the input-channel will be persisted to the
Message Store identified with the message-store attribute and indexed with generated ID. That ID is the
Claim Check for that Message.
The Claim Check will also become the payload of the new (transformed) Message that will be sent to the output-channel.

Now, lets assume that at some point you do need access to the actual Message. You can of course access the Message Store
manually and get the contents of the Message, or you can use the same approach as before except now you will be transforming
the Claim Check to the actual Message by using an Outgoing Claim Check Transformer.

Outgoing Claim Check Transformer

An Outgoing Claim Check Transformer allows you to transform a Message with a Claim Check payload
into a Message with the original content as its payload.

In the above configuration, the Message that is received on the input-channel should have a Claim Check as its payload
and the Outgoing Claim Check Transformer will transform it into a Message with the original payload by simply
querying the Message store for a Message identified by the provided Claim Check. It then sends the newly checked-out Message to the
output-channel.

Claim Once

There are scenarios when a particular message must be claimed only once. As an analogy, consider the airplane luggage check-in/out process.
Checking-in your luggage on the departure and and then claiming it on the arrival is a classic example of such a scenario.
Once the luggage was claimed it can not be claimed again without first checking it back in. To accommodate such cases we
introduced a remove-message boolean attribute on the claim-check-out transformer. This attribute is
set to false by default. However if set to true, the claimed Message will also be removed
from the MessageStore so that it can no longer be claimed again. This is also something to consider in terms of storage space,
especially in the case of the in-memory Map-based SimpleMessageStore where failing to remove the Messages
could ultimately lead to an OutOfMemoryException. If you don't expect multiple claims to be made, it's
recommended that you set the remove-message attribute's value to false.

Although we rarely care about the details of the claim checks as long as they work, it is still worth knowing that
the current implementation of the actual Claim Check (the pointer) in Spring Integration is a UUID to ensure uniqueness.

A word on Message Store

org.springframework.integration.store.MessageStore is a strategy interface for storing and retrieving messages.
Spring Integration provides two convenient implementations of it. SimpleMessageStore: an in-memory, Map-based
implementation (the default, good for testing) and JdbcMessageStore: an implementation that uses a relational
database via JDBC.

7. Messaging Endpoints

7.1 Message Endpoints

The first part of this chapter covers some background theory and reveals quite a bit about the underlying API
that drives Spring Integration's various messaging components. This information can be helpful if you want to
really understand what's going on behind the scenes. However, if you want to get up and running with the
simplified namespace-based configuration of the various elements, feel free to skip ahead to
Section 7.1.4, “Namespace Support” for now.

As mentioned in the overview, Message Endpoints are responsible for connecting the various messaging components to
channels. Over the next several chapters, you will see a number of different components that consume Messages. Some
of these are also capable of sending reply Messages. Sending Messages is quite straightforward. As shown above in
Section 3.1, “Message Channels”, it's easy to send a Message to a Message Channel. However,
receiving is a bit more complicated. The main reason is that there are two types of consumers:
Polling Consumers and
Event Driven Consumers.

Of the two, Event Driven Consumers are much simpler. Without any need to manage and schedule a separate poller
thread, they are essentially just listeners with a callback method. When connecting to one of Spring Integration's
subscribable Message Channels, this simple option works great. However, when connecting to a buffering, pollable
Message Channel, some component has to schedule and manage the polling thread(s). Spring Integration provides
two different endpoint implementations to accommodate these two types of consumers. Therefore, the consumers
themselves can simply implement the callback interface. When polling is required, the endpoint acts as a
"container" for the consumer instance. The benefit is similar to that of using a container for hosting
Message Driven Beans, but since these consumers are simply Spring-managed Objects running within an
ApplicationContext, it more closely resembles Spring's own MessageListener containers.

7.1.1 Message Handler

Spring Integration's MessageHandler interface is implemented by many of the
components within the framework. In other words, this is not part of the public API, and a developer would not
typically implement MessageHandler directly. Nevertheless, it is used by a Message
Consumer for actually handling the consumed Messages, and so being aware of this strategy interface does help in
terms of understanding the overall role of a consumer. The interface is defined as follows:

Despite its simplicity, this provides the foundation for most of the components that will be covered in the
following chapters (Routers, Transformers, Splitters, Aggregators, Service Activators, etc). Those components
each perform very different functionality with the Messages they handle, but the requirements for actually
receiving a Message are the same, and the choice between polling and event-driven behavior is also the same.
Spring Integration provides two endpoint implementations that "host" these callback-based handlers and allow
them to be connected to Message Channels.

7.1.2 Event Driven Consumer

Because it is the simpler of the two, we will cover the Event Driven Consumer endpoint first. You may recall that
the SubscribableChannel interface provides a subscribe()
method and that the method accepts a MessageHandler parameter (as shown in
the section called “SubscribableChannel”):

subscribableChannel.subscribe(messageHandler);

Since a handler that is subscribed to a channel does not have to actively poll that channel, this is an
Event Driven Consumer, and the implementation provided by Spring Integration accepts a
a SubscribableChannel and a MessageHandler:

Spring Integration currently provides two implementations of the Trigger
interface: IntervalTrigger and CronTrigger. The
IntervalTrigger is typically defined with a simple interval (in milliseconds), but
also supports an 'initialDelay' property and a boolean 'fixedRate' property (the default is false, i.e.
fixed delay):

The 'maxMessagesPerPoll' property specifies the maximum number of messages to receive within a given poll
operation. This means that the poller will continue calling receive() without waiting
until either null is returned or that max is reached. For example, if a poller has a 10 second
interval trigger and a 'maxMessagesPerPoll' setting of 25, and it is polling a channel that has 100 messages
in its queue, all 100 messages can be retrieved within 40 seconds. It grabs 25, waits 10 seconds, grabs the
next 25, and so on.

The 'receiveTimeout' property specifies the amount of time the poller should wait if no messages are
available when it invokes the receive operation. For example, consider two options that seem similar on
the surface but are actually quite different: the first has an interval trigger of 5 seconds and a receive
timeout of 50 milliseconds while the second has an interval trigger of 50 milliseconds and a receive timeout
of 5 seconds. The first one may receive a message up to 4950 milliseconds later than it arrived on the channel
(if that message arrived immediately after one of its poll calls returned). On the other hand, the second
configuration will never miss a message by more than 50 milliseconds. The difference is that the second
option requires a thread to wait, but as a result it is able to respond much more quickly to arriving messages.
This technique, known as "long polling", can be used to emulate event-driven behavior on a polled source.

A Polling Consumer may also delegate to a Spring TaskExecutor, and it can
be configured to participate in Spring-managed transactions. The following example shows the configuration of both:

The examples above show dependency lookups, but keep in mind that these consumers will most often be configured
as Spring bean definitions. In fact, Spring Integration also provides a
FactoryBean that creates the appropriate consumer type based on the type of
channel, and there is full XML namespace support to even further hide those details. The namespace-based
configuration will be featured as each component type is introduced.

Note

Many of the MessageHandler implementations are also capable of generating reply
Messages. As mentioned above, sending Messages is trivial when compared to the Message reception. Nevertheless,
when and how many reply Messages are sent depends on the handler
type. For example, an Aggregator waits for a number of Messages to arrive and is often
configured as a downstream consumer for a Splitter which may generate multiple
replies for each Message it handles. When using the namespace configuration, you do not strictly need to know
all of the details, but it still might be worth knowing that several of these components share a common base
class, the AbstractReplyProducingMessageHandler, and it provides a
setOutputChannel(..) method.

7.1.4 Namespace Support

Throughout the reference manual, you will see specific configuration examples for endpoint elements, such as
router, transformer, service-activator, and so on. Most of these will support an "input-channel" attribute and
many will support an "output-channel" attribute. After being parsed, these endpoint elements produce an instance
of either the PollingConsumer or the
EventDrivenConsumer depending on the type of the "input-channel" that is
referenced: PollableChannel or SubscribableChannel
respectively. When the channel is pollable, then the polling behavior is determined based on the endpoint
element's "poller" sub-element and its attributes. For example, a simple interval-based poller with a 1-second interval would be
configured like this:

If the input channel is a PollableChannel, then the poller configuration is
required. Specifically, as mentioned above, the 'trigger' is a required property of the PollingConsumer class.
Therefore, if you omit the "poller" sub-element for a Polling Consumer endpoint's configuration, an Exception
may be thrown. The exception will also be thrown if you attempt to configure a poller on the element that is
connected to a non-pollable channel.

It is also possible to create top-level pollers in which case only a "ref" is required:

The "ref" attribute is only allowed on the inner-poller definitions. Defining this attribute on a top-level
poller will result in a configuration exception thrown during initialization of the Application Context.

In fact, to simplify the configuration even further, you can define a global default poller. A single top-level poller within
an ApplicationContext may have the default attribute with a value of "true". In that case, any
endpoint with a PollableChannel for its input-channel that is defined within the same ApplicationContext and has
no explicitly configured 'poller' sub-element will use that default.

<pollerid="defaultPoller"default="true"max-messages-per-poll="5"fixed-rate="3000"/><!-- No <poller/> sub-element is necessary since there is a default --><transformerinput-channel="pollable"ref="transformer"output-channel="output"/>

Spring Integration also provides transaction support for the pollers so that each receive-and-forward
operation can be performed as an atomic unit-of-work. To configure transactions for a poller, simply add the
<transactional/> sub-element. The attributes for this element should be familiar to anyone who has
experience with Spring's Transaction management:

Since Spring transaction support depends on the Proxy mechanism with TransactionInterceptor (AOP Advice) handling transactional
behavior of the message flow initiated by the poler, some times there is a need to provide extra Advice(s) to handle other
cross cutting behavior associated with the poller. For that poller defines an 'advice-chain' element allowing you to add
more advices - class that implements MethodInterceptor interface..

For more information on how to implement MethodInterceptor please refer to AOP sections of Spring
reference manual (section 7 and 8). Advice chain can also be applied on the poller that does not have
any transaction configuration essentially allowing you to enhance the behavior of the message flow initiated by the poller.

The polling threads may be executed by any instance of Spring's TaskExecutor
abstraction. This enables concurrency for an endpoint or group of endpoints. As of Spring 3.0, there is a "task"
namespace in the core Spring Framework, and its <executor/> element supports the creation of a simple thread
pool executor. That element accepts attributes for common concurrency settings such as pool-size and queue-capacity.
Configuring a thread-pooling executor can make a substantial difference in how the endpoint performs under load. These
settings are available per-endpoint since the performance of an endpoint is one of the major factors to consider
(the other major factor being the expected volume on the channel to which the endpoint subscribes). To enable
concurrency for a polling endpoint that is configured with the XML namespace support, provide the 'task-executor'
reference on its <poller/> element and then provide one or more of the properties shown below:

If no 'task-executor' is provided, the consumer's handler will be invoked in the caller's thread. Note that the
"caller" is usually the default TaskScheduler
(see Section B.3, “Configuring the Task Scheduler”). Also, keep in mind that the 'task-executor' attribute can
provide a reference to any implementation of Spring's TaskExecutor interface by
specifying the bean name. The "executor" element above is simply provided for convenience.

As mentioned in the background section for Polling Consumers above, you can also configure a Polling Consumer
in such a way as to emulate event-driven behavior. With a long receive-timeout and a short interval-trigger,
you can ensure a very timely reaction to arriving messages even on a polled message source. Note that this
will only apply to sources that have a blocking wait call with a timeout. For example, the File poller does
not block, each receive() call returns immediately and either contains new files or not. Therefore, even if
a poller contains a long receive-timeout, that value would never be usable in such a scenario. On the other
hand when using Spring Integration's own queue-based channels, the timeout value does have a chance to
participate. The following example demonstrates how a Polling Consumer will receive Messages nearly
instantaneously.

Using this approach does not carry much overhead since internally it is nothing more then a timed-wait thread
which does not require nearly as much CPU resource usage as a thrashing, infinite while loop for example.

7.1.5 Payload Type Conversion

Throughout the reference manual, you will also see specific configuration and implementation examples of various endpoints
which can accept a Message or any arbitrary Object as an input parameter. In the case of an Object, such a parameter will
be mapped to a Message payload or part of the payload or header (when using the Spring Expression Language). However there
are times when the type of input parameter of the endpoint method does not match the type of the payload or its part.
In this scenario we need to perform type conversion. Spring Integration provides a convenient way for registering type
converters (using the Spring 3.x ConversionService) within its own instance of a conversion service bean named integrationConversionService.
That bean is automatically created as soon as the first converter is defined using the Spring Integration namespace support.
To register a Converter all you need is to implement
org.springframework.core.convert.converter.Converter and define it via
convenient namespace support:

7.1.6 Asynchronous polling

If you want the polling to be asynchronous, a Poller can optionally specify a 'task-executor' attribute
pointing to an existing instance of any TaskExecutor bean
(Spring 3.0 provides a convenient namespace configuration via the task namespace). However, there are certain things
you must understand when configuring a Poller with a TaskExecutor.

The problem is that there are two configurations in place. The Poller and the TaskExecutor,
and they both have to be in tune with each other otherwise you might end up creating an artificial memory leak.

Let's look at the following configuration provided by one of the users on the Spring Integration
forum (http://forum.springsource.org/showthread.php?t=94519):

The above configuration demonstrates one of those out of tune configurations.

The poller keeps scheduling new tasks even though all the threads are blocked waiting for either a new message to arrive,
or the timeout to expire. Given that there are 20 threads executing tasks with a 5 second timeout, they will be executed
at a rate of 4 per second (5000/20 = 250ms). But, new tasks are being scheduled at a rate of 20 per second, so the internal
queue in the task executor will grow at a rate of 16 per second (while the process is idle), so we essentially have a memory leak.

One of the ways to handle this is to set the queue-capacity attribute of the Task Executor to 0. You can also
manage it by specifying what to do with messages that can not be queued by setting the rejection-policy attribute
of the Task Executor (e.g., DISCARD). In other words there are certain details you must understand with regard to configuring
the TaskExecutor. Please refer to - Section 25 - Task Execution and Scheduling of the Spring reference manual
for more detail on the subject.

7.2 Inbound Messaging Gateways

7.2.1 GatewayProxyFactoryBean

Working with Objects instead of Messages is an improvement. However, it would be even better to have no
dependency on the Spring Integration API at all - including the gateway class. For that reason, Spring
Integration also provides a GatewayProxyFactoryBean that generates a proxy for
any interface and internally invokes the gateway methods shown below.

With this configuration defined, the "cafeService" can now be injected into other beans, and the code that invokes the methods on that
proxied instance of the Cafe interface has no awareness of the Spring Integration API. The general
approach is similar to that of Spring Remoting (RMI, HttpInvoker, etc.). See the "Samples" Appendix for
an example that uses this "gateway" element (in the Cafe demo).

Important

Typically you don't have to specify the default-reply-channel since a Gateway will
auto-create a temporary, anonymous reply channel where it will listen for the reply.
However, there are some cases which may prompt you to define a default-reply-channel (or reply-channel
with adapter gateways such as HTTP, JMS, etc.).
For some background, we'll quickly discuss some of the inner-workings of the Gateway.
A Gateway will create a temporary point-to-point reply channel which is anonymous and is added
to the Message Headers with the name replyChannel.
When providing an explicit default-reply-channel (reply-channel with remote adapter gateways),
you have the option to point to a publish-subscribe channel, which is so named because you can add more than one subscriber to it.
Internally Spring Integration will create a Bridge between the temporary replyChannel and the explicitly defined
default-reply-channel.
So let's say you want your reply to go not only to the gateway, but also to some other consumer. In this case you
would want two things: a) a named channel you can subscribe to and b) that channel is a publish-subscribe-channel.
The default strategy used by the gateway will not satisfy those needs, because the reply channel added to the header is anonymous and
point-to-point. This means that no other subscriber can get a handle to it and even if it could, the channel
has point-to-point behavior such that only one subscriber would get the Message. So by defining a default-reply-channel
you can point to a channel of your choosing which in this case would be a publish-subscribe-channel.
The Gateway would create a bridge from it to the temporary, anonymous reply channel that is stored in the header.
Another case where you might want to provide a reply channel explicitly is for monitoring or auditing via an interceptor
(e.g., wiretap). You need a named channel in order to configure a Channel Interceptor.

The reason that the attributes on the 'gateway' element are named 'default-request-channel' and
'default-reply-channel' is that you may also provide per-method channel references by using the
@Gateway annotation.

You can also provide individual headers per method invocation via XML.
This could be very useful if the headers you want to set are static in nature and you don't want
to embed them in the gateway's method signature via @Header annotations.
For example, in the Loan Broker example we want to influence how aggregation of the Loan quotes
will be done based on what type of request was initiated (single quote or all quotes). Determining the
type of the request by evaluating what gateway method was invoked, although possible would
violate the separation of concerns paradigm (the method is a java artifact), but expressing your
intention (meta information) via Message headers is natural in a Messaging architecture.

In the above case you can clearly see how a different value will be set for the 'RESPONSE_TYPE'
header based on the gateway's method.

Of course, the Gateway invocation might result in errors.
By default any error that has occurred downstream will be re-thrown as a MessagingException (RuntimeException)
upon the Gateway's method invocation. However there are times when you may want
to simply log the error rather than propagating it, or you may want to treat an
Exception as a valid reply, by mapping it to a Message that will conform to some
"error message" contract that the caller understands. To accomplish this, our
Gateway provides support for a Message Channel dedicated to the errors via the
error-channel attribute. In the example below, you can see
that a 'transformer' is used to create a reply Message from the Exception.

The exceptionTransformer could be a simple POJO that
knows how to create the expected error response objects. That would then be
the payload that is sent back to the caller. Obviously, you could do many
more elaborate things in such an "error flow" if necessary. It might involve
routers (including Spring Integration's ErrorMessageExceptionTypeRouter),
filters, and so on. Most of the time, a simple 'transformer' should be sufficient,
however.

Alternatively, you might want to only log the Exception (or send it somewhere
asynchronously). If you provide a one-way flow, then nothing would be sent
back to the caller. In the case that you want to completely suppress Exceptions,
you can provide a reference to the global "nullChannel" (essentially a /dev/null
approach). Finally, as mentioned above, if no "error-channel" is defined at all,
then the Exceptions will propagate as usual.

Important

Exposing the messaging system via simple POJI Gateways obviously provides benefits, but "hiding" the reality
of the underlying messaging system does come at a price so there are certain things you should consider.
We want our Java method to return as quickly as possible and not hang for an indefinite amount of time while
the caller is waiting on it to return (void, return value, or a thrown Exception). When regular methods are
used as a proxies in front of the Messaging system, we have to take into account the potentially asynchronous
nature of the underlying messaging. This means that there might
be a chance that a Message that was initiated by a Gateway could be dropped by a Filter, thus never reaching a
component that is responsible for producing a reply. Some Service Activator method might result in an Exception,
thus providing no reply (as we don't generate Null messages). So as you can see there are multiple scenarios
where a reply message might not be coming. That is perfectly natural in messaging systems. However think about the
implication on the gateway method. The Gateway's method input arguments were incorporated into a Message and
sent downstream. The reply Message would be converted to a return value of the Gateway's method. So you might want
to ensure that for each Gateway call there will always be a reply Message.
Otherwise, your Gateway method might never return and will hang indefinitely.
One of the ways of handling this situation is via an Asynchronous Gateway (explained later in this section). Another way of handling it is to explicitly set the reply-timeout attribute. That way, the gateway will not hang any longer than the time specified by the reply-timeout and will return 'null' if that timeout does elapse. Finally, you might want to consider setting downstream flags such as 'requires-reply' on
a service-activator or 'throw-exceptions-on-rejection' on a filter. These options will be discussed in more detail in the final section
of this chapter.

7.2.2 Asynchronous Gateway

As a pattern the Messaging Gateway is a very nice way to hide messaging-specific code while still exposing the full capabilities of the
messaging system. As you've seen, the GatewayProxyFactoryBean provides a convenient way to expose a Proxy over a service-interface
thus giving you POJO-based access to a messaging system (based on objects in your own domain, or primitives/Strings, etc). But when a
gateway is exposed via simple POJO methods which return values it does imply that for each Request message (generated when the method is invoked)
there must be a Reply message (generated when the method has returned). Since Messaging systems naturally are asynchronous you may not always be
able to guarantee the contract where "for each request there will always be be a reply".
With Spring Integration 2.0 we are introducing support for an Asynchronous Gateway which is a convenient way to initiate
flows where you may not know if a reply is expected or how long will it take for replies to arrive.

A natural way to handle these types of scenarios in Java would be relying upon java.util.concurrent.Future instances, and
that is exactly what Spring Integration uses to support an Asynchronous Gateway.

From the XML configuration, there is nothing different and you still define Asynchronous Gateway the same way as a regular Gateway.

As you can see from the example above the return type for the gateway method is a Future. When
GatewayProxyFactoryBean sees that the
return type of the gateway method is a Future, it immediately switches to the async mode by utilizing
an AsyncTaskExecutor. That is all. The call to such a method always returns immediately with a Future instance.
Then, you can interact with the Future at your own pace to get the result, cancel, etc. And, as with
any other use of Future instances, calling get() may reveal a timeout, an execution exception, and so on.

For a more detailed example, please refer to the async-gateway sample distributed within the Spring Integration samples.

7.2.3 Gateway behavior when no response arrives

As it was explained earlier, the Gateway provides a convenient way of interacting with a Messaging system via POJO method
invocations, but realizing that a typical method invocation, which is generally expected to always return (even with an Exception),
might not always map one-to-one to message exchanges (e.g., a reply message might not arrive - which is equivalent to a
method not returning). It is important to go over several scenarios especially in the Sync Gateway case and understand
the default behavior of the Gateway and how to deal with these scenarios to make the Sync Gateway behavior more
predictable regardless of the outcome of the message flow that was initialed from such Gateway.

There are certain attributes that could be configured to make Sync Gateway behavior more predictable,
but some of them might not always work as you might have expected. One of them is reply-timeout.
So, lets look at the reply-timeout attribute and see how it can/can't influence the behavior
of the Sync Gateway in various scenarios. We will look at single-threaded scenario
(all components downstream are connected via Direct Channel) and multi-threaded scenarios
(e.g., somewhere downstream you may have Pollable or Executor Channel which breaks single-thread boundary)

Long running process downstream

Sync Gateway - single-threaded.
If a component downstream is still running (e.g., infinite loop or a very slow service), then setting a reply-timeout
has no effect and the Gateway method call will not return until such downstream service exits (via return or exception).
Sync Gateway - multi-threaded.
If a component downstream is still running (e.g., infinite loop or a very slow service), in a multi-threaded message
flow setting the reply-timeout will have an effect by allowing gateway method invocation to
return once the timeout has been reached, since the GatewayProxyFactoryBean will simply
poll on the reply channel waiting for a message until the timeout expires. However it could result in a 'null' return
from the Gateway method if the timeout has been reached before the actual reply was produced. It is also important to understand that
the reply message (if produced) will be sent to a reply channel after the Gateway method invocation might have returned, so you must be aware of that and design your flow with this in mind.

Downstream component returns 'null'

Sync Gateway - single-threaded.
If a component downstream returns 'null' and no reply-timeout has been configured, the Gateway
method call will hang indefinitely unless: a) a reply-timeout has been configured or b) the
requires-reply attribute has been set on the downstream component (e.g., service-activator)
that might return 'null'. In this case, an Exception would be thrown and propagated to the Gateway.
Sync Gateway - multi-threaded. Behavior is the same as above.

Sync Gateway - single-threaded.
If a component downstream returns 'void' and no reply-timeout has been configured,
the Gateway method call will hang indefinitely unless a reply-timeout has been configured
Sync Gateway - multi-threaded Behavior is the same as above.

Sync Gateway - single-threaded.
If a component downstream throws a Runtime Exception, such exception will be propagated via an Error Message back to
the gateway and re-thrown.
Sync Gateway - multi-threaded Behavior is the same as above.

Important

It is also important to understand that by default reply-timeout is unbounded* which means that
if not explicitly set there are several scenarios (described above) where your Gateway method invocation might
hang indefinitely. So, make sure you analyze your flow and if there is even a remote possibility of one of these
scenarios to occur, set the reply-timeout attribute to a 'safe' value or, even better,
set the requires-reply attribute of the downstream component to 'true' to ensure a timely response
as produced by the throwing of an Exception as soon as that downstream component does return null internally.
But also, realize that there are some scenarios (see the very first one)
where reply-timeout will not help. That means it is also important to analyze your message
flow and decide when to use a Sync Gateway vs an Async Gateway. As you've seen the latter case is simply a matter of
defining Gateway methods that return Future instances. Then, you are guaranteed to receive that return value, and
you will have more granular control over the results of the invocation.

Also, when dealing with a Router you should remember that setting the resolution-required attribute to 'true'
will result in an Exception thrown by the router if it can not resolve a particular channel. Likewise, when dealing with a Filter,
you can set the throw-exception-on-rejection attribute. In both of these cases, the resulting flow will
behave like that containing a service-activator with the 'requires-reply' attribute. In other words, it will help to ensure
a timely response from the Gateway method invocation.

Note

* reply-timeout is unbounded for <gateway/>
elements (created by the GatewayProxyFactoryBean). Inbound gateways for external integration
(ws, http, etc.) share many characteristics and attributes with these gateways. However,
for those inbound gateways, the default reply-timeout is 1000
milliseconds (1 second). If a downstream async handoff is made to another thread, you may need to
increase this attribute to allow enough time for the flow to complete before the
gateway times out.

7.3 Service Activator

7.3.1 Introduction

The Service Activator is the endpoint type for connecting any Spring-managed Object to an input channel so that
it may play the role of a service. If the service produces output, it may also be connected to an output channel.
Alternatively, an output producing service may be located at the end of a processing pipeline or message flow in
which case, the inbound Message's "replyChannel" header can be used. This is the default behavior if no output
channel is defined, and as with most of the configuration options you'll see here, the same behavior actually
applies for most of the other components we have seen.

7.3.2 Configuring Service Activator

To create a Service Activator, use the 'service-activator' element with the 'input-channel' and 'ref' attributes:

The configuration above assumes that "exampleHandler" either contains a single method annotated with the
@ServiceActivator annotation or that it contains only one public method at all. To delegate to an explicitly
defined method of any object, simply add the "method" attribute.

In either case, when the service method returns a non-null value, the endpoint will attempt to send the reply
message to an appropriate reply channel. To determine the reply channel, it will first check if an
"output-channel" was provided in the endpoint configuration:

If no "output-channel" is available, it will then check the Message's replyChannel header
value. If that value is available, it will then check its type. If it is a
MessageChannel, the reply message will be sent to that channel. If it is a
String, then the endpoint will attempt to resolve the channel name to a channel instance.
If the channel cannot be resolved, then a ChannelResolutionException will be thrown.
It it can be resolved, the Message will be sent there. This is the technique used for Request Reply messaging
in Spring Integration, and it is also an example of the Return Address pattern.

The argument in the service method could be either a Message or an arbitrary type. If the latter, then it will
be assumed that it is a Message payload, which will be extracted from the message and injected into such service
method. This is generally the recommended approach as it follows and promotes a POJO model when working with Spring
Integration. Arguments may also have @Header or @Headers annotations as described in Section B.5, “Annotation Support”

Note

The service method is not required to have any arguments at all, which means you
can implement event-style Service Activators, where all you care about is an invocation of the service method,
not worrying about the contents of the message. Think of it as a NULL JMS message. An example use-case for such an
implementation could be a simple counter/monitor of messages deposited on the input channel.

Using a "ref" attribute is generally recommended if the custom Service Activator handler implementation can be reused
in other <service-activator> definitions. However if the custom Service Activator handler implementation
is only used within a single definition of the <service-activator>, you can provide an inner bean definition:

Using both the "ref" attribute and an inner handler definition in the same <service-activator>
configuration is not allowed, as it creates an ambiguous condition and will result in an Exception being thrown.

Service Activators and the Spring Expression Language (SpEL)

Since Spring Integration 2.0, Service Activators can also benefit from SpEL
(http://static.springsource.org/spring/docs/3.0.x/spring-framework-reference/html/expressions.html).

For example, you may now invoke any bean method without pointing to the bean via a ref attribute or including it as an
inner bean definition. For example:

In the above configuration instead of injecting 'accountService' using a ref or as an inner bean,
we are simply using SpEL's @beanId notation and invoking a method which takes a type compatible with Message payload. We
are also passing a header value. As you can see, any valid SpEL expression can be evaluated against any content in the Message.
For simple scenarios your Service Activators do not even have to reference a bean if all logic can be encapsulated
by such an expression.

In the above configuration our service logic is to simply multiply the payload value by 2, and SpEL lets us handle it relatively easy.

7.4 Delayer

7.4.1 Introduction

A Delayer is a simple endpoint that allows a Message flow to be delayed by a certain interval. When
a Message is delayed, the original sender will not block. Instead, the delayed Messages will be
scheduled with an instance of java.util.concurrent.ScheduledExecutorService
to be sent to the output channel after the delay has passed. This approach is scalable even for
rather long delays, since it does not result in a large number of blocked sender Threads. On the
contrary, in the typical case a thread pool will be used for the actual execution of releasing the
Messages. Below you will find several examples of configuring a Delayer.

7.4.2 Configuring Delayer

The <delayer> element is used to delay the Message flow between two Message Channels.
As with the other endpoints, you can provide the "input-channel" and "output-channel" attributes,
but the delayer also requires at least the 'default-delay' attribute with the number of milliseconds
that each Message should be delayed.

In the example above the 3 second delay would only apply in the case that the header value is
not present for a given inbound Message. If you only want to apply a delay to Messages that have
an explicit header value, then you can set the 'default-delay' to 0. For any Message that has a
delay of 0 (or less), the Message will be sent directly. In fact, if there is not a positive delay
value for a Message, it will be sent to the output channel on the calling Thread.

Tip

The delay handler actually supports header values that represent an interval in milliseconds (any
Object whose toString() method produces a value that can be parsed into a
Long) as well as java.util.Date instances representing an absolute time.
In the former case, the milliseconds will be counted from the current time (e.g. a value of 5000
would delay the Message for at least 5 seconds from the time it is received by the Delayer). In
the latter case, with an actual Date instance, the Message will not be released until that Date
occurs. In either case, a value that equates to a non-positive delay, or a Date in the past, will
not result in any delay. Instead, it will be sent directly to the output channel in the original
sender's Thread.

The delayer delegates to an instance of Spring's TaskScheduler abstraction.
The default scheduler used by the delayer is a ThreadPoolTaskScheduler instance with a pool size of 1.
If you want to delegate to a different scheduler, you can provide a reference through the delayer element's
'scheduler' attribute:

7.5 Groovy support

With Spring Integration 2.0 we've added Groovy support allowing you to
use the Groovy scripting language to provide the logic for
various integration components similar to the way the Spring Expression Language
(SpEL) is supported for routing, transformation and other integration
concerns. For more information about Groovy please refer to the Groovy
documentation which you can find
on the project website

7.5.1 Groovy configuration

Depending on the complexity of your integration requirements Groovy
scripts could be provided inline as CDATA in XML configuration or as a
reference to a file containing the Groovy script. To enable Groovy support
Spring Integration defines a
GroovyScriptExecutingMessageProcessor which will
bind the Message Payload as a
payload variable and the Message Headers as a headers
variable within the script execution context. All that is left for you to do is
write a script that uses those
variables. Below are a couple of sample configurations:

Here, you see that the script can be included inline
or via the location attribute using the groovy namespace
support.

Other supported elements are router, service-activator,
transformer, and splitter. The configuration would look identical to that
above other than the main element's name.

Another interesting aspect of using Groovy support is the framework's
ability to update (reload) scripts without restarting the Application
Context. To accomplish this, all you need to do is specify
the refresh-check-delay attribute on the script
element.

<groovy:script location="..."refresh-check-delay="5000"/>

In the above example any invocations that occur within the 5 seconds immediately following the
updating of the script would still be using the old script. However, any invocation that occurs
after those 5 seconds have elapsed will
result in execution of the new script. This is a good example where 'near real
time' is acceptable.

<groovy:script location="..."refresh-check-delay="0"/>

In the above example the context will be updated with any script modifications
as soon as such modification occurs. Basically this is an example of
'real-time' configuration and might not be the most efficient option (but could be useful during development).

<groovy:script location="..."refresh-check-delay="-1"/>

Any negative number value means the script will never be refreshed after
initial initialization of the application context. This is the default behavior.
In this case, the "dynamic" aspect of Groovy is not being used, but the syntax
might be the primary reason that Groovy has been chosen in the first place.

Important

Inline defined scripts can not be reloaded.

Custom bindings

You already know that by default, 'payload' and 'headers' will be bound as Groovy binding variables.
However, some times in order to take the most out of Groovy you may want to customize Groovy bindings
(e.g., include extra variables pointing to some scalar values or bind some beans as variables).
To support this requirement we have defined a simple strategy: ScriptVariableGenerator.

As you can see the only method to implement is generateScriptVariables(Message). It takes the Message
as an argument. That allows you to use data available in the Message payload and/or headers. The return value is
the Map of variables that will be bound to the script's evaluation context. This method will be called every time
the script is executed, corresponding to the processing of that particular Message. We also provide a
default implementation and namespace based configuration for simple bindings via <variable> sub-elements (see below):

As you can see similar to other constructs in Spring, when binding each of these variables you can either provide a
scalar value or reference another bean in the Application Context.

If you need more control over how a particular variable is generated, then all you need to do is
provide your own implementation of ScriptVariableGenerator and reference it with the script-variable-generator
attribute:

The script-variable-generator attribute and <variable> sub-element(s) are mutually exclusive.
You can use at most one of them. Also, the script-variable-generator and <variable> sub-elements
cannot be used with an inline script, but rather only when pointing to the script via the location attribute.

If you need to customize the Groovy object itself, beyond setting variables, you can reference
a bean that implementats org.springframework.scripting.groovy.GroovyObjectCustomizer via the
customizer attribute. For example, this might be useful if you want to configure a domain-specific
language (DSL) by modifying the MetaClass and registering functions to be available within the script.

Setting a custom GroovyObjectCustomizer is not mutually exclusive with <variable> sub-elements or
the script-variable-generator attribute. It can also be provided when defining an inline script.

7.5.2 Control Bus

As described in (EIP), the idea
behind the Control Bus is that the same messaging system can be used for
monitoring and managing the components within the framework as is used for
"application-level" messaging. In Spring Integration we build upon the
adapters described above so that it's possible to send Messages as a means
of invoking exposed operations. One option for those operations is Groovy scripts.

<groovy:control-businput-channel="operationChannel"/>

The Control Bus has an input channel that can be accessed for
invoking operations on the beans in the application context.

The Groovy Control Bus executes messages on the input channel as
Groovy scripts. It takes a message, compiles the body to a Script,
customizes it with a GroovyObjectCustomizer, and then executes it. The
Control Bus' customizer exposes all the beans in the application context
that are annotated with @ManagedResource, implement Spring's
Lifecycle interface or extend Spring's CustomizableThreadCreator base class
(e.g. several of the TaskExecutor and TaskScheduler implementations).

If you need to further customize the Groovy objects, you can also provide a reference to a bean
that implements org.springframework.scripting.groovy.GroovyObjectCustomizer via
the customizer attribute.

8.1.1 Notification Listening Channel Adapter

The Notification-listening Channel Adapter requires a JMX ObjectName
for the MBean that publishes Notifications to which this listener should
be registered. A very simple configuration might look like this:

The notification-listening-channel-adapter
registers with an MBeanServer at startup, and the default bean name is
"mbeanServer" which happens to be the same bean name generated when using
Spring's <context:mbean-server/> element. If you need to use a
different name be sure to include the "mbean-server" attribute.

The
adapter can also accept a reference to a NotificationFilter and a
"handback" Object to provide some context that is passed back with each
Notification. Both of those attributes are optional. Extending the above
example to include those attributes as well as an explicit MBeanServer
bean name would produce the following:

It does also require that an MBeanExporter be present in the context. That
is why the <context:mbean-export/> element is shown above as
well.

When Messages are sent to the channel for this adapter, the
Notification is created from the Message content. If the payload is a
String it will be passed as the "message" text for the Notification. Any
other payload type will be passed as the "userData" of the
Notification.

JMX Notifications also have a "type", and it should be a
dot-delimited String. There are two ways to provide the type. Precedence
will always be given to a Message header value associated with the
JmxHeaders.NOTIFICATION_TYPE key. On the other hand, you can rely on a
fallback "default-notification-type" attribute provided in the
configuration.

8.1.3 Attribute Polling Channel Adapter

The attribute polling adapter is useful when you have a requirement
to periodically check on some value that is available through an MBean as
a managed attribute. The poller can be configured in the same way as any
other polling adapter in Spring Integration (or it's possible to rely on
the default poller). The "object-name" and "attribute-name" are required.
An MBeanServer reference is also required, but it will automatically check
for a bean named "mbeanServer" by default just like the
notification-listening-channel-adapter described above.

8.1.4 Operation Invoking Channel Adapter

The operation-invoking-channel-adapter enables
Message-driven invocation of any managed operation exposed by an MBean.
Each invocation requires the operation name to be invoked and the
ObjectName of the target MBean. Both of these must be explicitly provided
via adapter configuration:

Then the adapter
only needs to be able to discover the "mbeanServer" bean. If a different
bean name is required, then provide the "mbean-server" attribute with a
reference.

The payload of the Message will be mapped to the parameters of the
operation, if any. A Map-typed payload with String keys is treated as
name/value pairs whereas a List or array would be passed as a simple
argument list (with no explicit parameter names). If the operation
requires a single parameter value, then the payload can represent that
single value, and if the operation requires no parameters, then the
payload would be ignored.

If you want to expose a channel for a single common operation to be
invoked by Messages that need not contain headers, then that option works
well.

8.1.5 Operation Invoking outbound Gateway

Similar to operation-invoking-channel-adapter
Spring Integration also provides
operation-invoking-outbound-gateway which could be
used when dealing with non-void operations and return value is required.
Such return value will be sent as message payload to the 'reply-channel'
specified by this Gateway.

Another way of
provideing the 'reply-channel' is by setting
MessageHeaders.REPLY_CHANNEL Message
Header

8.1.6 MBean Exporter

Spring Integration components themselves may be exposed as MBeans
when the IntegrationMBeanExporter is configured. To
create an instance of the IntegrationMBeanExporter,
define a bean and provide a reference to an MBeanServer and a domain name
(if desired). The domain can be left out in which case the default domain
is "org.springframework.integration".

Then start
JConsole (free with the JDK), and connect to the local process on
localhost:6969 to get a look at the management
endpoints exposed. (The port and client are just examples to get you
started quickly, there are other JMX clients available and some offer more
sophisticated features than JConsole.)

The MBean exporter is orthogonal to the one provided in Spring core
- it registers message channels and message handlers, but not itself. You
can expose the exporter itself, and certain other components in Spring
Integration, using the standard
<context:mbean-export/> tag. The exporter has a
couple of useful metrics attached to it, for instance a count of the
number of active handlers and the number of queued messages (these would
both be important if you wanted to shutdown the context without losing any
messages).

MBean ObjectNames

All the MessageChannel, MessageHandler and MessageSource instances
in the application are wrapped by the MBean exporter to provide
management and monitoring features. For example, MessageChannel send The
generated JMX object names for each component type are listed in the
table below

The "bean" attribute in the object names for sources
and handlers takes one of the values in the table below

Table 8.2.

Bean Value

Description

endpoint

The bean name of the enclosing endpoint (e.g.
<service-activator>) if there is one

anonymous

An indication that the enclosing endpoint didn't have a
user-specified bean name, so the JMX name is the input channel
name

internal

For well-known Spring Integration default
components

handler

None of the above: fallback to the
toString() of the object being monitored
(handler or source)

MessageChannel MBean Features

Message channels report metrics according to their concrete type.
If you are looking at a DirectChannel you will
see statistics for the send operation. If it is a
QueueChannel you will also see statistics for the
receive operation. In both cases there are some metrics that are simple
counters (message count and error count), and some that are estimates of
averages of interesting quantities. The algorithms used to calculate
these estimates are described briefly in the table below:

Table 8.3.

Metric Type

Example

Algorithm

Count

Send Count

Simple incrementer. Increase by one when an event
occurs.

Duration

Send Duration (method execution time in
milliseconds)

Exponential Moving Average with decay factor 10. Average
of the method execution time over roughly the last 10
measurements.

Rate

Send Rate (number of operations per second)

Inverse of Exponential Moving Average of the interval
between events with decay in time (lapsing over 60 seconds) and
per measurement (last 10 events).

Ratio

Send Error Ratio (ratio of errors to total sends)

Estimate the success ratio as the Exponential Moving
Average of the series composed of values 1 for success and 0 for
failure (decaying as per the rate measurement over time and
events). Error ratio is 1 - success ratio.

A feature of the time-based average estimates is that they decay
with time if no new measurements arrive. To help interpret the behaviour
over time, the time (in seconds) since the last measurement is also
exposed as a metric.

There are two basic exponential models: decay per measurement
(appropriate for duration and anything where the number of measurements
is part of the metric), and decay per time unit (more suitable for rate
measurements where the time in between measurements is part of the
metric). Both models depend on the fact that

S(n) = sum(i=0,i=n) w(i) x(i)

has a special form when w(i) = r^i, with
r=constant:

S(n) = x(n) + r S(n-1)

(so
you only have to store S(n-1), not the whole series
x(i), to generate a new metric estimate from the last
measurement). The algorithms used in the duration metrics use
r=exp(-1/M) with M=10. The net
effect is that the estimate S(n) is more heavily
weighted to recent measurements and is composed roughly of the last
M measurements. So M is the
"window" or lapse rate of the estimate In the case of the vanilla moving
average, i is a counter over the number of
measurements. In the case of the rate we interpret i
as the elapsed time, or a combination of elapsed time and a counter (so
the metric estimate contains contributions roughly from the last
M measurements and the last T
seconds).

8.2 Message History

The key benefit of a messaging architecture is loose coupling where participating components do not maintain any awareness about one another. This fact
alone makes your application extremely flexible, allowing you to change components without affecting the rest of the flow, change messaging routes,
message consuming styles (polling vs event driven), and so on.
However, this unassuming style of architecture could prove to be difficult when things go wrong. When debugging,
you would probably like to get as much information about the message as you can (its origin, channels it has traversed, etc.)

Message History is one of those patterns that helps by giving you an option to maintain some level of awareness of a
message path either for debugging purposes or to maintain an audit trail.
Spring integration provides a simple way to configure your message flows to maintain the Message History by adding a header to the
Message and updating that header every time a message passes through a tracked component.

8.2.1 Message History Configuration

To enable Message History all you need is to define the message-history element in your configuration.

<int:message-history/>

Now every named component (component that has an 'id' defined) will be tracked.
The framework will set the 'history' header in your Message. Its value is very simple - List<Properties>.

You might not want to track all of the components. To limit the history to certain components based on their names,
all you need is provide the tracked-components attribute and specify
a comma-delimited list of component names and/or patterns that match the components you want to track.

<int:message-historytracked-components="*Gateway, sample*, foo"/>

In the above example, Message History will only be maintained for all of the components that end with 'Gateway', start with 'sample',
or match the name 'foo' exactly.

Note

Remember that by definition the Message History header is immutable (you can't re-write history, although some try). Therefore, when writing
Message History values, the components are either creating brand new Messages (when the component is an origin), or they are copying the
history from a request Message, modifying it and setting the new list on a reply Message. In either case, the values can be appended even
if the Message itself is crossing thread boundaries. That means that the history values can greatly simplify debugging in an
asynchronous message flow.

8.3 Control Bus

As described in (EIP), the idea behind the Control Bus is that the
same messaging system can be used for monitoring and managing the components
within the framework as is used for "application-level" messaging. In Spring
Integration we build upon the adapters described above so that it's possible
to send Messages as a means of invoking exposed operations.

<control-businput-channel="operationChannel"/>

The Control Bus has an input channel that can be accessed for invoking
operations on the beans in the application context. It also has all the
common properties of a service activating endpoint, e.g. you can specify an
output channel if the result of the operation has a return value that you
want to send on to a downstream channel.

The Control Bus executes messages on the input channel as Spring
Expression Language expressions. It takes a message, compiles the body to an
expression, adds some context, and then executes it. The default context
supports any method that has been annotated with @ManagedAttribute or @ManagedOperation.
It also supports the methods on Spring's Lifecycle interface, and it supports methods
that are used to configure several of Spring's TaskExecutor and TaskScheduler implementations.
The simplest way to ensure that your own methods are available to the Control Bus is to
use the @ManagedAttribute and/or @ManagedOperation annotations. Since those are also used
for exposing methods to a JMX MBean registry, it's a convenient by-product (often the same
types of operations you want to expose to the Control Bus would be reasonable for exposing
via JMS). Resolution of any particular instance within the application context is achieved
in the typical SpEL syntax. Simply provide the bean name with the SpEL prefix for beans (@).
For example, to execute a method on a Spring Bean a client
could send a message to the operation channel as follows:

The root of the context for the expression is the
Message itself, so you also have access to the 'payload'
and 'headers' as variables within your expression. This is consistent with all the other expression
support in Spring Integration endpoints.

Part IV. Integration Adapters

This section covers the various Channel Adapters and Messaging Gateways provided
by Spring Integration to support Message-based communication with external systems.

9. Spring ApplicationEvent Support

Spring Integration provides support for inbound and outbound ApplicationEvents
as defined by the underlying Spring Framework. For more information about Spring's support for events and listeners,
refer to the Spring Reference Manual.

9.1 Receiving Spring ApplicationEvents

To receive events and send them to a channel, simply define an instance of Spring Integration's
ApplicationEventListeningMessageProducer. This class is an implementation of
Spring's ApplicationListener interface. By default it will pass all
received events as Spring Integration Messages. To limit based on the type of event, configure the
list of event types that you want to receive with the 'eventTypes' property. If a received event
has a Message instance as its 'source', then that will be passed as-is. Otherwise, if a SpEL-based
"payloadExpression" has been provided, that will be evaluated against the ApplicationEvent instance.
If the event's source is not a Message instance and no "payloadExpression" has been provided, then
the ApplicationEvent itself will be passed as the payload.

For convenience namespace support is provided to configure an ApplicationEventListeningMessageProducer
via the inbound-channel-adapter element.

In the above example, all Application Context events that match one of the types specified by the 'event-types' (optional) attribute will be
delivered as Spring Integration Messages to the Message Channel named 'eventChannel'. If a downstream component throws
an exception, a MessagingException containing the failed message and exception will be sent to the channel named
'eventErrorChannel'. If no "error-channel" is specified and the downstream channels are synchronous, the Exception
will be propagated to the caller.

9.2 Sending Spring ApplicationEvents

To send Spring ApplicationEvents, create an instance of the
ApplicationEventPublishingMessageHandler and register it within an endpoint.
This implementation of the MessageHandler interface also implements
Spring's ApplicationEventPublisherAware interface and thus acts as a
bridge between Spring Integration Messages and ApplicationEvents.

For convenience namespace support is provided to configure an ApplicationEventPublishingMessageHandler
via the outbound-channel-adapter element.

If you are using a PollableChannel (e.g., Queue), you can also provide poller as a sub-element of the
outbound-channel-adapter element. You can also optionally provide a task-executor
reference for that poller. The following example demonstrates both.

In the above example, all messages sent to the 'eventChannel' channel will be published as ApplicationEvents to any relevant
ApplicationListener instances that are registered within the same Spring ApplicationContext. If the payload of the Message is
an ApplicationEvent, it will be passed as-is. Otherwise the Message itself will be wrapped in a MessagingEvent instance.

10. Feed Adapter

Spring Integration provides support for Syndication via Feed Adapters

10.1 Introduction

Web syndication is a form of publishing material such as news stories, press releases, blog posts, and
other items typically available on a website but also made available in a feed format such as RSS or ATOM.

Spring integration provides support for Web Syndication via its 'feed' adapter and provides convenient
namespace-based configuration for it.
To configure the 'feed' namespace, include the following elements within the headers of your XML configuration file:

In the above configuration, we are subscribing to a URL identified by the url attribute.

As news items are retrieved they will be converted to Messages and sent to a channel identified by the channel attribute.
The payload of each message will be a com.sun.syndication.feed.synd.SyndEntry instance. That encapsulates
various data about a news item (content, dates, authors, etc.).

You can also see that the Inbound Feed Channel Adapter is a Polling Consumer. That means you have to
provide a poller configuration. However, one important thing you must understand with regard to Feeds is that its inner-workings
are slightly different then most other poling consumers. When an Inbound Feed adapter is started, it does the first poll and
receives a com.sun.syndication.feed.synd.SyndEntryFeed instance. That is an object that contains multiple
SyndEntry objects. Each entry is stored in the local entry queue and is released based on
the value in the max-messages-per-poll attribute such that each Message will contain a single entry.
If during retrieval of the entries from the entry queue the queue had become empty, the adapter will attempt to update
the Feed thereby populating the queue with more entries (SyndEntry instances) if available. Otherwise the next attempt to
poll for a feed will be determined by the trigger of the poller (e.g., every 10 seconds in the above configuration).

Duplicate Entries

Polling for a Feed might result in entries that have already been processed
("I already read that news item, why are you showing it to me again?").
Spring Integration provides a convenient mechanism to eliminate the need to worry about duplicate entries.
Each feed entry will have a published date field. Every time a new Message is generated and sent,
Spring Integration will store the value of the latest published date in an instance of the
org.springframework.integration.store.MetadataStore strategy. The MetadataStore interface is
designed to store various types of generic meta-data (e.g., published date of the last feed entry that has been processed)
to help components such as this Feed adapter deal with duplicates.

The default rule for locating this metadata store is as follows: Spring Integration will look for a bean of type
org.springframework.integration.store.MetadataStore in the ApplicationContext. If one is found then it will be used,
otherwise it will create a new instance of SimpleMetadataStore which is an in-memory implementation that
will only persist metadata within the lifecycle of the currently running Application Context. This means that upon restart you may
end up with duplicate entries. If you need to persist metadata between Application Context restarts, you may use the
PropertiesPersistingMetadataStore which is backed by a properties file and a properties-persister.
Alternatively, you could provide your own implementation of the MetadataStore interface
(e.g. JdbcMetadataStore) and configure it as bean in the Application Context.

11. File Support

11.1 Introduction

Spring Integration's File support extends the Spring Integration Core with
a dedicated vocabulary to deal with reading, writing, and transforming files.
It provides a namespace that enables elements defining Channel Adapters dedicated
to files and support for Transformers that can read file contents into strings or
byte arrays.

This section will explain the workings of FileReadingMessageSource
and FileWritingMessageHandler and how to configure them as
beans. Also the support for dealing with files through file specific
implementations of Transformer will be discussed. Finally the
file specific namespace will be explained.

11.2 Reading Files

A FileReadingMessageSource can be used to consume files from the filesystem.
This is an implementation of MessageSource that creates messages from
a file system directory.

A common problem with reading files is that a file may be detected before
it is ready. The default AcceptOnceFileListFilter
does not prevent this. In most cases, this can be prevented if the
file-writing process renames each file as soon as it is ready for
reading. A filename-pattern or filename-regex filter that accepts only files that are
ready (e.g. based on a known suffix), composed with the default
AcceptOnceFileListFilter allows for this.
The CompositeFileListFilter enables the
composition.

The first channel adapter is relying on the default filter that just prevents
duplication, the second is using a custom filter, the third is using the
filename-pattern attribute to add an AntPathMatcher
based filter, and the fourth is using the filename-regex attribute to add a
regular expression Pattern based filter to the FileReadingMessageSource.
The filename-pattern and filename-regex attributes are
each mutually exclusive with the regular filter reference attribute. However,
you can use the filter attribute to reference an instance of
CompositeFileListFilter that combines any number of filters, including one
or more pattern based filters to fit your particular needs.

When multiple processes are reading from the same directory it can be desirable to lock files to prevent
them from being picked up concurrently. To do this you can use a FileLocker.
There is a java.nio based implementation available out of the box, but it is also possible to implement your
own locking scheme. The nio locker can be injected as follows

When a file inbound adapter is configured with a locker, it will take the responsibility to acquire a
lock before the file is allowed to be received.
<strong>It will not assume the responsibility to unlock the file.</strong>
If you have processed the file and keeping the locks hanging around you have a memory leak. If this is
a problem in your case you should call FileLocker.unlock(File file) yourself at the appropriate time.

When filtering and locking files is not enough it might be needed to control the way files are listed entirely. To
implement this type of requirement you can use an implementation of DirectoryScanner.
This scanner allows you to determine entirely what files are listed each poll. This is also the interface
that Spring Integration uses internally to wire FileListFilters FileLocker to the FileReadingMessageSource.
A custom DirectoryScanner can be injected into the <file:inbound-channel-adapter/> on the scanner
attribute.

This gives you full freedom to choose the ordering, listing and locking strategies.

11.3 Writing files

To write messages to the file system you can use a
FileWritingMessageHandler. This class can deal with
File, String, or byte array payloads. In its simplest form the
FileWritingMessageHandler only requires a
destination directory for writing the files. The name of the file to be
written is determined by the handler's FileNameGenerator.
The default implementation looks for a Message header whose key matches
the constant defined as FileHeaders.FILENAME.

Additionally, you can configure the encoding and the charset that
will be used in case of a String payload.

To make things easier you can configure the FileWritingMessageHandler as
part of an outbound channel adapter using the namespace.

The namespace based configuration also supports a delete-source-files attribute.
If set to true, it will trigger deletion of the original source files after writing
to a destination. The default value for that flag is false.

The delete-source-files attribute will only have an effect if the inbound
Message has a File payload or if the FileHeaders.ORIGINAL_FILE header
value contains either the source File instance or a String representing the original file path.

In cases where you want to continue processing messages based on the written File you can use
the outbound-gateway instead. It plays a very similar role as the
outbound-channel-adapter. However after writing the File, it will also send it
to the reply channel as the payload of a Message.

The 'outbound-gateway' works well in cases where you want to first move a File and then send it
through a processing pipeline. In such cases, you may connect the file namespace's
'inbound-channel-adapter' element to the 'outbound-gateway' and then connect that gateway's
reply-channel to the beginning of the pipeline.

If you have more elaborate requirements or need to support additional payload types as input
to be converted to file content you could extend the FileWritingMessageHandler, but a much
better option is to rely on a Transformer.

11.4 File Transformers

To transform data read from the file system to objects and the other way around you need
to do some work. Contrary to FileReadingMessageSource and to a
lesser extent FileWritingMessageHandler, it is very likely that you
will need your own mechanism to get the job done. For this you can implement the
Transformer interface. Or extend the
AbstractFilePayloadTransformer for inbound messages. Some obvious
implementations have been provided.

FileToByteArrayTransformer transforms Files into byte[]s using
Spring's FileCopyUtils. It is often better to use a sequence of
transformers than to put all transformations in a single class. In that case the File to
byte[] conversion might be a logical first step.

FileToStringTransformer will convert Files to Strings as the name
suggests. If nothing else, this can be useful for debugging (consider using with a Wire Tap).

To configure File specific transformers you can use the appropriate elements from the file namespace.

The delete-files option signals to the transformer that it should delete
the inbound File after the transformation is complete. This is in no way a replacement for using the
AcceptOnceFileListFilter when the FileReadingMessageSource is being used in a
multi-threaded environment (e.g. Spring Integration in general).

12. FTP/FTPS Adapters

12.1 Introduction

The File Transfer Protocol (FTP) is a simple network protocol which allows you to transfer files between two computers on the Internet.

There are two actors when it comes to FTP communication: client and server.
To transfer files with FTP/FTPS, you use a client which initiates a connection to a remote computer
that is running an FTP server. After the connection is established, the client can choose
to send and/or receive copies of files.

Spring Integration supports sending and receiving files over FTP/FTPS by providing two types of client
side adapters: Inbound Channel Adapter and Outbound Channel Adapter. It also provides
convenient namespace-based configuration options for defining these client components.

To use the FTP namespace, add the following to the header of your XML file:

12.2 FTP Session Factory

Before configuring FTP adapters you must configure an FTP Session Factory. You can configure
the FTP Session Factory with a regular bean definition where the implementation class is org.springframework.integration.ftp.session.DefaultFtpSessionFactory:
Below is a basic configuration:

Every time an adapter requests a session object from its SessionFactory the session is
returned from a session pool maintained by a caching wrapper around the factory. A Session in the session pool might go stale
(if it has been disconnected by the server due to inactivity) so the SessionFactory
will perform validation to make sure that it never returns a stale session to the adapter. If a stale session was encountered,
it will be removed from the pool, and a new one will be created.

Note

If you experience connectivity problems and would like to trace Session creation as well as see which Sessions are
polled you may enable it by setting the logger to TRACE level (e.g., log4j.category.org.springframework.integration.file=TRACE)

Now all you need to do is inject these session factories into your adapters. Obviously the protocol (FTP or FTPS) that an adapter will
use depends on the type of session factory that has been injected into the adapter.

Note

A more practical way to provide values for FTP/FTPS Session Factories is by using Spring's property
placeholder support (See: http://static.springsource.org/spring/docs/3.0.x/spring-framework-reference/html/beans.html#beans-factory-placeholderconfigurer).

Advanced Configuration

DefaultFtpSessionFactory provides an abstraction over the underlying client API which, in the current release of
Spring Integration, is Apache Commons Net. This spares you from the low level configuration details
of the org.apache.commons.net.ftp.FTPClient. However there are times when access to lower level FTPClient details is
necessary to achieve more advanced configuration (e.g., setting data timeout, default timeout etc.). For that purpose, AbstractFtpSessionFactory
(the base class for all FTP Session Factories) exposes hooks, in the form of the two post-processing methods below.

As you can see, there is no default implementation for these two methods. However, by extending DefaultFtpSessionFactory you can override these methods
to provide more advanced configuration of the FTPClient. For example:

12.3 FTP Inbound Channel Adapter

The FTP Inbound Channel Adapter is a special listener that will connect to the FTP server and will listen
for the remote directory events (e.g., new file created) at which point it will initiate a file transfer.

As you can see from the configuration above you can configure an FTP Inbound Channel Adapter via the inbound-channel-adapter
element while also providing values for various attributes such as local-directory, filename-pattern
(which is based on simple pattern matching, not regular expressions), and of course the reference to a session-factory.

Some times file filtering based on the simple pattern specified via filename-pattern attribute might not be
sufficient. If this is the case, you can use the filename-regex attribute to specify a Regular Expression
(e.g. filename-regex=".*\.test$"). And of course if you need complete control you can use filter
attribute and provide a reference to any custom implementation of the
org.springframework.integration.file.filters.FileListFilter, a strategy interface for filtering a
list of files.

Note

As of Spring Integration 2.0.2, we have added a 'remote-file-separator' attribute. That allows you to configure a
file separator character to use if the default '/' is not applicable for your particular environment.

Please refer to the schema for more details on these attributes.

It is also important to understand that the FTP Inbound Channel Adapter is a Polling Consumer and
therefore you must configure a poller (either via a global default or a local sub-element).
Once a file has been transferred, a Message with a java.io.File as its payload will be generated and sent to the channel
identified by the channel attribute.

More on File Filtering and Large Files

Some times the file that just appeared in the monitored (remote) directory is not complete. Typically such a file
will be written with temporary extension (e.g., foo.txt.writing) and then renamed after the writing process finished.
As a user in most cases you are only interested in files that are complete and would like to filter only files that are complete.
To handle these scenarios you can use the filtering support provided by the filename-pattern, filename-regex
and filter attributes. Here is an example that uses a custom Filter implementation.

The job of the inbound FTP adapter consists of two tasks:
1) Communicate with a remote server in order to transfer files from a remote directory to a local directory.2) For each transferred file, generate a Message with that file as a payload and send it to the channel identified by the 'channel' attribute.
That is why they are called 'channel-adapters' rather than just 'adapters'. The main job of such an adapter is to generate a
Message to be sent to a Message Channel. Essentially, the second task mentioned above takes precedence in such a way that
*IF* your local directory already has one or more files it will first generate Messages from those, and *ONLY*
when all local files have been processed, will it initiate the remote communication to retrieve more files.

Also, when configuring a trigger on the poller you should pay close attention to the max-messages-per-poll
attribute. Its default value is 1 for all SourcePollingChannelAdapter instances (including FTP).
This means that as soon as one file is processed, it will wait for the next execution time as determined by your
trigger configuration. If you happened to have one or more files sitting in the local-directory, it would process
those files before it would initiate communication with the remote FTP server. And, if the max-messages-per-poll
were set to 1 (default), then it would be processing only one file at a time with intervals as defined by your trigger,
essentially working as one-poll = one-file.

For typical file-transfer use cases, you most likely want the opposite behavior: to process all the files you can for each
poll and only then wait for the next poll. If that is the case, set max-messages-per-poll to -1. Then, on
each poll, the adapter will attempt to generate as many Messages as it possibly can. In other words, it will process
everything in the local directory, and then it will connect to the remote directory to transfer everything that is available
there to be processed locally. Only then is the poll operation considered complete, and the poller will wait for the next execution time.

You can alternatively set the 'max-messages-per-poll' value to a positive value indicating the upward limit of Messages to be created
from files with each poll. For example, a value of 10 means that on each poll it will attempt to process no more than 10 files.

12.4 FTP Outbound Channel Adapter

The FTP Outbound Channel Adapter relies upon a MessageHandler implementation that will connect to the
FTP server and initiate an FTP transfer for every file it receives in the payload of incoming Messages. It also supports several
representations of the File so you are not limited only to java.io.File typed payloads.
The FTP Outbound Channel Adapter
supports the following payloads: 1) java.io.File - the actual file object;
2) byte[] - a byte array that represents the file contents; and 3) java.lang.String -
text that represents the file contents.

As you can see from the configuration above you can configure an FTP Outbound Channel Adapter via the
outbound-channel-adapter element while also providing values for various attributes such as filename-generator
(an implementation of the org.springframework.integration.file.FileNameGenerator strategy interface),
a reference to a session-factory, as well as other attributes. You can also see
some examples of *expression attributes which allow you to use SpEL
to configure things like remote-directory-expression, temporary-remote-directory-expression and remote-filename-generator-expression
(a SpEL alternative to filename-generator shown above). As with any component that allows the usage of SpEL, access to Payload and Message Headers is available via
'payload' and 'headers' variables.
Please refer to the schema for more details on
the available attributes.

Note

By default Spring Integration will use org.springframework.integration.file.DefaultFileNameGenerator if none is specified.
DefaultFileNameGenerator will determine the file name based on the value of the file_name header (if it exists)
in the MessageHeaders, or if the payload of the Message is already a java.io.File, then it will use the original name of that file.

Important

Defining certain values (e.g., remote-directory) might be platform/ftp server dependent. For example as it
was reported on this forum http://forum.springsource.org/showthread.php?p=333478&posted=1#post333478 on some
platforms you must add slash to the end of the directory definition (e.g., remote-directory="/foo/bar/"
instead of remote-directory="/foo/bar")

12.5 FTP Session Caching

One of the optimizations implemented by the FTP adapters is session caching. Similar to JDBC pooling of Connections, the FTP Adapters maintain a
pool of Sessions by default. However there are times when this behavior is not desired (e.g., security etc.).
To disable session caching you can set the cache-sessions attribute to false (the default value is true).

13. HTTP Support

13.1 Introduction

The HTTP support allows for the execution of HTTP requests and the processing of inbound HTTP requests. Because interaction over HTTP is always synchronous, even if all that is returned is a 200 status code, the HTTP support consists of two gateway implementations:
HttpInboundEndpoint and HttpRequestExecutingMessageHandler.

13.2 Http Inbound Gateway

To receive messages over HTTP you need to use an HTTP inbound Channel Adapter or Gateway. In common with the HttpInvoker
support the HTTP inbound adapters need to be deployed within a servlet container. The easiest way to do this is to provide a servlet
definition in web.xml, see
Section 14.2, “HttpInvoker Inbound Gateway” for further details. Below is an example bean definition for a simple HTTP inbound endpoint.

The HttpRequestHandlingMessagingGateway accepts a list of HttpMessageConverter instances or else
relies on a default list. The converters allow
customization of the mapping from HttpServletRequest to Message. The default converters
encapsulate simple strategies, which for
example will create a String message for a POST request where the content type starts with "text", see the Javadoc for
full details.

Starting with this release MultiPart File support was implemented. If the request has been wrapped as a
MultipartHttpServletRequest, when using the default converters, that request will be converted
to a Message payload that is a MultiValueMap containing values that may be byte arrays, Strings, or instances of
Spring's MultipartFile depending on the content type of the individual parts.

Note

The HTTP inbound Endpoint will locate a MultipartResolver in the context if one exists with the bean name
"multipartResolver" (the same name expected by Spring's DispatcherServlet). If it does in fact locate that
bean, then the support for MultipartFiles will be enabled on the inbound request mapper. Otherwise, it will
fail when trying to map a multipart-file request to a Spring Integration Message. For more on Spring's
support for MultipartResolvers, refer to the Spring Reference Manual.

In sending a response to the client there are a number of ways to customize the behavior of the gateway. By default the gateway will
simply acknowledge that the request was received by sending a 200 status code back. It is possible to customize this response by providing a
'viewName' to be resolved by the Spring MVC ViewResolver.
In the case that the gateway should expect a reply to the Message then setting the expectReply flag
(constructor argument) will cause
the gateway to wait for a reply Message before creating an HTTP response. Below is an example of a gateway
configured to serve as a Spring MVC Controller with a view name. Because of the constructor arg value of TRUE, it wait for a reply. This also shows
how to customize the HTTP methods accepted by the gateway, which
are POST and GET by default.

The reply message will be available in the Model map. The key that is used
for that map entry by default is 'reply', but this can be overridden by setting the
'replyKey' property on the endpoint's configuration.

13.3 Http Outbound Gateway

To configure the HttpRequestExecutingMessageHandler write a bean definition like this:

This bean definition will execute HTTP requests by delegating to a RestTemplate. That template in turn delegates
to a list of HttpMessageConverters to generate the HTTP request body from the Message payload. You can configure those converters as well
as the ClientHttpRequestFactory instance to use:

By default the HTTP request will be generated using an instance of SimpleClientHttpRequestFactory which uses the JDK
HttpURLConnection. Use of the Apache Commons HTTP Client is also supported through the provided
CommonsClientHttpRequestFactory which can be injected as shown above.

Note

In the case of the Outbound Gateway, the reply message produced by the gateway will contain all Message Headers present in the request message.

13.4 HTTP Namespace Support

Spring Integration provides an "http" namespace and schema definition. To include it in your
configuration, simply provide the following URI within a namespace declaration:
'http://www.springframework.org/schema/integration/http'. The schema location should then map to
'http://www.springframework.org/schema/integration/http/spring-integration-http.xsd'.

To configure an inbound http channel adapter which is an instance of HttpInboundEndpoint configured
not to expect a response.

To configure the outbound gateway you can use the namespace support as well. The following code snippet shows the different configuration options for an outbound Http gateway. Most importantly, notice that the 'http-method' and 'expected-response-type' are provided. Those are two of the most commonly configured values. The
default http-method is POST, and the default response type is null. With a null response type, the payload of the reply Message would only
contain the status code (e.g. 200) as long as it's a successful status (non-successful status codes will throw Exceptions). If you are expecting a different
type, such as a String, then provide that fully-qualified class name as shown below.

If your outbound adapter is to be used in a unidirectional way, then you can use an outbound-channel-adapter instead. This means that
a successful response will simply execute without sending any Messages to a reply channel. In the case of any non-successful response
status code, it will throw an exception. The configuration looks very similar to the gateway:

The uri-variable defines two attributes expression and value. You generally use
the value attribute for literal values, but if the value you are trying to inject is dynamic and requires
access to Message data you can use a SpEL expression via the expression attribute. In the above configuration
the getZip() method will be invoked on the payload object of the Message and the result of that
method will be used as the value for URI variable named 'zipCode'.

13.5 HTTP Proxy configuration

If you are behind a proxy and need to configure proxy settings for HTTP outbound adapters and/or
gateways, you can apply one of two approaches. In most cases, you can rely on the standard Java
System Properties that control the proxy settings. Otherwise, you can explicitly configure a
Spring bean for the HTTP client request factory instance.

Standard Java Proxy configuration

There are 3 System Properties you can set to configure the proxy settings that will be used by the HTTP protocol handler:

http.proxyHost - the host name of the proxy server.

http.proxyPort - the port number, the default value being 80.

http.nonProxyHosts - a list of hosts that should be reached directly,
bypassing the proxy. This is a list of patterns separated by '|'. The patterns may start or end with a '*' for wildcards. Any host matching one of these patterns will be reached through a direct connection instead of through a proxy.

And for HTTPS:

https.proxyHost - the host name of the proxy server.

https.proxyPort - the port number, the default value being 80.

For more information please refer to this document: http://download.oracle.com/javase/6/docs/technotes/guides/net/proxies.html

Spring's SimpleClientHttpRequestFactory

If for any reason, you need more explicit control over the proxy configuration, you can use Spring's
SimpleClientHttpRequestFactory and configure its 'proxy' property as such:

13.6 HTTP Header Mappings

Spring Integration provides support for Http Header mapping for both HTTP Request and HTTP Responses.

By default all standard Http Headers as defined here
http://en.wikipedia.org/wiki/List_of_HTTP_header_fields will be mapped from the message to HTTP request/response headers without
further configuration.
However if you do need further customization you may provide additional configuration via convenient namespace support.
You can provide a comma-separated list of header names, and you can also include simple patterns with the '*' character acting as a wildcard.
If you do provide such values, it will override the default behavior. Basically, it assumes you are in complete control at that point.
However, if you do want to include all of the standard HTTP headers, you can use the shortcut patterns: HTTP_REQUEST_HEADERS and
HTTP_RESPONSE_HEADERS. Here are some examples:

The adapters and gateways will use the DefaultHttpHeaderMapper which now provides
two static factory methods for "inbound" and "outbound" adapters so that the proper direction can be
applied (mapping HTTP requests/responses IN/OUT as appropriate).

If further customization is required you can also configure a DefaultHttpHeaderMapper independently
and inject it into the adapter via the header-mapper attribute.

13.7 HTTP Samples

This example demonstrates how simple it is to send a Multipart HTTP request via Spring's RestTemplate and receive
it with a Spring Integration HTTP Inbound Adapter. All we are doing is creating a MultiValueMap and
populating it with multi-part data. The RestTemplate will take care of the rest (no pun intended) by
converting it to a MultipartHttpServletRequest . This particular client will send a multipart HTTP Request
which contains the name of the company as well as an image file with the company logo.

14. HttpInvoker Support

14.1 Introduction

Important

This module is DEPRECATED and will be removed in future releases of Spring Integration. Please use HTTP support (Chapter 13, HTTP Support) which should
provide all the functionality that was available with Http Invoker plus more.

HttpInvoker is a Spring-specific remoting option that essentially enables Remote Procedure Calls (RPC) over HTTP.
In order to accomplish this, an outbound representation of a method invocation is serialized using standard Java
serialization and then passed within an HTTP POST request. After being invoked on the target system, the method's
return value is then serialized and written to the HTTP response. There are two main requirements. First, you
must be using Spring on both sides since the marshalling to and from HTTP requests and responses is handled by
the client-side invoker and server-side exporter. Second, the Objects that you are passing must implement
Serializable and be available on both the client and server.

While traditional RPC provides physical decoupling, it does not offer nearly the same degree
of logical decoupling as a messaging-based system. In other words, both participants in an
RPC-based invocation must be aware of a specific interface and specific argument types. Interestingly, in Spring
Integration, the "parameter" being sent is a Spring Integration Message, and the interface is an internal detail
of Spring Integration's implementation. Therefore, the RPC mechanism is being used as a
transport so that from the end user's perspective, it is not necessary to consider the
interface and argument types. It's just another adapter to enable messaging between two systems.

14.2 HttpInvoker Inbound Gateway

To receive messages over http you can use an HttpInvokerInboundGateway. Here is an
example bean definition:

If you are running within a Spring MVC application and using the BeanNameHandlerMapping, then the servlet
definition is not necessary. In that case, the bean name for your gateway can be matched against the URL
path just like a Spring MVC Controller bean.

14.3 HttpInvoker Outbound Gateway

To configure the HttpInvokerOutboundGateway write a bean definition like this:

The outbound gateway is a MessageHandler and can therefore be registered with
either a PollingConsumer or EventDrivenConsumer.
The URL must match that defined by an inbound HttpInvoker Gateway as described in the previous section.

14.4 HttpInvoker Namespace Support

Spring Integration provides an "httpinvoker" namespace and schema definition. To include it in your
configuration, simply provide the following URI within a namespace declaration:
'http://www.springframework.org/schema/integration/httpinvoker'. The schema location should then map to
'http://www.springframework.org/schema/integration/httpinvoker/spring-integration-httpinvoker-2.0.xsd'.

To configure the inbound gateway you can choose to use the namespace support for it. The following code snippet shows the different configuration options that are supported.

A 'reply-channel' may also be provided, but it is recommended to rely on the temporary anonymous channel
that will be created automatically for handling replies.

To configure the outbound gateway you can use the namespace support as well. The following code snippet shows the different configuration for an outbound HttpInvoker gateway. Only the 'url' and 'request-channel' are required.

MailSendingMessageHandler has various mapping strategies that use Spring's
MailMessage abstraction. If the received Message's payload is already
a MailMessage instance, it will be sent directly.
Therefore, it is generally recommended to precede this
consumer with a Transformer for non-trivial MailMessage construction requirements. However, a few simple
Message mapping strategies are supported out-of-the-box. For example, if the message payload is a byte array,
then that will be mapped to an attachment. For simple text-based emails, you can provide a String-based
Message payload. In that case, a MailMessage will be created with that String as the text content. If you
are working with a Message payload type whose toString() method returns appropriate mail text content, then
consider adding Spring Integration's ObjectToStringTransformer prior to the outbound
Mail adapter (see the example within the section called “Configuring Transformer with XML” for more detail).

The outbound MailMessage may also be configured with certain values from the
MessageHeaders. If available, values will be mapped to the outbound mail's
properties, such as the recipients (TO, CC, and BCC), the from/reply-to, and the subject. The header names are
defined by the following constants:

MailHeaders also allows you to override corresponding MailMessage values.
For example: If MailMessage.to is set to '[email protected]' and MailHeaders.TO
Message header is provided it will take precedence and override the corresponding value in MailMessage

15.2 Mail-Receiving Channel Adapter

Spring Integration also provides support for inbound email with the
MailReceivingMessageSource. It delegates to a configured instance of Spring
Integration's own MailReceiver interface, and there are two implementations:
Pop3MailReceiver and ImapMailReceiver. The easiest way to
instantiate either of these is by passing the 'uri' for a Mail store to the receiver's constructor. For example:

Another option for receiving mail is the IMAP "idle" command (if supported by the mail server you are using).
Spring Integration provides the ImapIdleChannelAdapter which is itself a Message-producing
endpoint. It delegates to an instance of the ImapMailReceiver but enables asynchronous
reception of Mail Messages. There are examples in the next section of configuring both types of inbound Channel
Adapter with Spring Integration's namespace support in the 'mail' schema.

15.3 Mail Namespace Support

Spring Integration provides a namespace for mail-related configuration. To use it, configure the following schema
locations.

Keep in mind, as with any outbound Channel Adapter, if the referenced channel is a PollableChannel, a
<poller> sub-element should be provided with either an interval-trigger or cron-trigger.

To configure an Inbound Channel Adapter, you have the choice between polling or event-driven (assuming your
mail server supports IMAP IDLE - if not, then polling is the only option). A polling Channel Adapter simply
requires the store URI and the channel to send inbound Messages to. The URI may begin with "pop3" or "imap":

If you do have IMAP idle support, then you may want to configure the "imap-idle-channel-adapter" element instead.
Since the "idle" command enables event-driven notifications, no poller is necessary for this adapter. It will
send a Message to the specified channel as soon as it receives the notification that new mail is available:

When using IMAP IDLE channel adapter there might be situations where connection to the server may be lost
(e.g., network failure) and since Java Mail documentation explicitly states that the actual IMAP API is EXPERIMENTAL it is
important to understand the differences in the API and how to deal with them when configuring IMAP IDLE adapters.
Currently Spring Integration Mail adapters was tested with Java Mail 1.4.1 and Java Mail 1.4.3 and depending on which one
is used special attention must be payed to some of the java mail properties that needs to be set with regard to auto-reconnect.

The following behavior was observed with GMAIL but should provide you with some tips on how to solve re-connect
issue with other providers, however feedback is always welcome. Again, below notes are based on GMAIL.

With Java Mail 1.4.1 if mail.imaps.timeout property is set for a relatively short period of time (e.g., ~ 5 min)
then IMAPFolder.idle() will throw FolderClosedException after this timeout.
However if this property is not set (should be indefinite) the behavior that was observed is that IMAPFolder.idle()
method never returns nor it throws an exception. It will however reconnect automatically if connection was lost for a short
period of time (e.g., under 10 min), but if connection was lost for a long period of time (e.g., over 10 min), then
IMAPFolder.idle() will not throw FolderClosedException nor it will re-establish connection
and will remain in the blocked state indefinitely, thus leaving you no possibility to reconnect without restarting the adapter.
So the only way to make re-connect to work with Java Mail 1.4.1 is to set mail.imaps.timeout property explicitly
to some value, but it also means that such value shoudl be relatively short (under 10 min) and the connection should be
re-estabished relatively quickly. Again, it may be different with other providers.
With Java Mail 1.4.3 there was significant improvements to the API ensuring that there will always be a condition which
will force IMAPFolder.idle() method to return via StoreClosedException or
FolderClosedException or simply return, thus allowing us to proceed with auto-reconnect.
Currently auto-reconnect will run infinitely making attempts to reconnect every 10 sec.

Important

In both configurations channel and should-delete-messages are the REQUIRED
attributes. The important thing to understand is why should-delete-messages is required.
The issue is with the POP3 protocol, which does NOT have any knowledge of messages that were READ. It can only know what's been read
within a single session. This means that when your POP3 mail adapter is running, emails are successfully consumed as as they become available during each poll
and no single email message will be delivered more then once. However, as soon as you restart your adapter and begin a new session
all the email messages that might have been retrieved in the previous session will be retrieved again. That is the nature of POP3. Some might argue
that should-delete-messages should be TRUE by default. In other words, there are two valid and mutually exclusive use cases
which make it very hard to pick a single "best" default. You may want to configure your adapter as the only email receiver in which
case you want to be able to restart such adapter without fear that messages that were delivered before will not be redelivered again.
In this case setting should-delete-messages to TRUE would make most sense. However, you may have another use case where
you may want to have multiple adapters that simply monitor email servers and their content. In other words you just want to 'peek but not touch'.
Then setting should-delete-messages to FALSE would be much more appropriate. So since it is hard to choose what should be
the right default value for the should-delete-messages attribute, we simply made it a required attribute, to be set by the user.
Leaving it up to the user also means, you will be less likely to end up with unintended behavior.

Note

When configuring a polling email adapter's should-mark-messages-as-read attribute,
be aware of the protocol you are configuring to retrieve messages. For example POP3 does not support this flag
which means setting it to either value will have no effect as messages will NOT be marked as read.

When using the namespace support, a header-enricher Message Transformer is also available.
This simplifies the application of the headers mentioned above to any Message prior to sending to the
Mail-sending Channel Adapter.

Finally, the <imap-idle-channel-adapter/> also accepts the 'error-channel' attribute.
If a downstream exception is thrown and an 'error-channel' is specified,
a MessagingException message containing the failed message and original exception, will be sent to this channel.
Otherwise, if the downstream channels are synchronous, any such exception will simply be logged as a
warning by the channel adapter.

15.4 Email Message Filtering

Very often you may encounter a requirement to filter incoming messages
(e.g., You want to only read emails that have 'Spring Integration' in the Subject line). This
could be easily accomplished by connecting Inbound Mail adapter with an expression-based Filter. Although it would work, there is a
downside to this approach. Since messages would be filtered after going through inbound mail adapter all such messages would be marked as read (SEEN) or
Un-read (depending on the value of should-mark-messages-as-read attribute). However in reality what would be more useful is to mark messages
as SEEN only if they passed the filtering criteria. This is very similar to looking at your email client while scrolling through all the
messages in the preview pane, but only flagging messages as SEEN that were actually opened and read.

In Spring Integration 2.0.4 we've introduced mail-filter-expression attribute on inbound-channel-adapter and
imap-idle-channel-adapter. This attribute allows you to provide an expression which is a combination of SpEL and Regular Expression.
For example if you would like to read only emails that contain 'Spring Integration' in the Subject line, you would configure mail-filter-expression attribute
like this this: mail-filter-expression="subject matches '(?i).*Spring Integration.*"

Since javax.mail.internet.MimeMessage is the root context of SpEL Evaluation Context, you can filter on any value available
through MimeMessage including the actual body of the message. This one is particularly important since reading the body of the message would
typically result in such message to be marked as SEEN by default, but since we now setting PEAK flag of every incomming message to 'true', only
messages that were explicitly marked as SEEN will be seen as read.

So in the below example only messages that match the filter expression will be output by this adapter and only those messages will be marked as SEEN.
In this case based on the mail-filter-expression only messages that contain 'Spring Integration' in the subject line will be
produced by this adapter.

Another reasonable question is what happens on the next poll, or idle event, or what happens when such adapter is restarted.
Will there be a potential duplication of massages to be filtered? In other words if on the last retrieval where you had 5 new messages and only 1 passed
the filter what would happen with the other 4. Would they go through the filtering logic again on the next poll or idle? After all they were not marked
as SEEN. The actual answer is no. They would not be subject of duplicate processing due to another flag (RECENT) that is set by the Email server and
is used by Spring Integration mail search filter. Folder implementations set this flag to indicate that this message is new to this folder, that is,
it has arrived since the last time this folder was opened. In other while our adapter may peek at the email it also lets the email server know that
such email was touched and therefore will be marked as RECENT by the email server.

16. TCP and UDP Support

Spring Integration provides Channel Adapters for receiving and sending messages over internet protocols. Both UDP
(User Datagram Protocol)
and TCP (Transmission Control Protocol) adapters are provided. Each adapter provides for one-way communication
over the underlying protocol.
In addition, simple inbound and outbound tcp gateways are provided. These are used when two-way communication is
needed.

16.1 Introduction

Two flavors each of UDP inbound and outbound channel adapters are provided UnicastSendingMessageHandler
sends a datagram packet to a single destination. UnicastReceivingChannelAdapter receives
incoming datagram packets. MulticastSendingMessageHandler sends (broadcasts) datagram packets to
a multicast address. MulticastReceivingChannelAdapter receives incoming datagram packets
by joining to a multicast address.

An inbound TCP gateway is provided; this allows for simple request/response processing. While
the gateway can support any number of connections, each connection can only process serially. The thread
that reads from the socket waits for, and sends, the response before reading again. If the connection factory
is configured for single use connections, the connection is closed after the socket times out.

An outbound TCP gateway is provided; this allows for simple request/response processing.
If the associated connection factory is configured for single use connections, a new connection is
immediately created for each new request. Otherwise, if the connection is in use,
the calling thread blocks on the connection until either a response is received or a timeout
or I/O error occurs.

The TCP and UDP inbound channel adapters, and the TCP inbound gateway, support the "error-channel" attribute.
This provides the same basic functionality as described in Section 7.2.1, “GatewayProxyFactoryBean”.

16.2 UDP Adapters

When setting multicast to true, provide the multicast address in the host
attribute.

UDP is an efficient, but unreliable protocol. Two attributes are added to improve reliability. When check-length is
set to true, the adapter precedes the message data with a length field (4 bytes in network byte order). This enables
the receiving side to verify the length of the packet received. If a receiving system uses a buffer that is too
short the contain the packet, the packet can be truncated. The length header provides a mechanism to detect this.

An outbound channel adapter that adds length checking to the datagram packets and waits for an acknowledgment.

Tip

Setting acknowledge to true implies the recipient of the packet can interpret the header added to the packet
containing acknowledgment data (host and port). Most likely, the recipient will be a Spring Integration inbound
channel adapter.

Tip

When multicast is true, an additional attribute min-acks-for-success specifies
how many acknowledgments must be received within the ack-timeout.

By default, reverse DNS lookups are done on inbound packets to convert IP addresses to
hostnames for use in message headers.
In environments where DNS is not configured, this can cause delays.
This default behavior can be overridden by setting the lookup-host
attribute to "false".

16.3 TCP Connection Factories

For TCP, the configuration of the underlying connection is provided using a
Connection Factory. Two types of connection factory are provided; a
client connection factory and a server connection factory. Client connection
factories are used to establish outgoing connections; Server connection factories
listen for incoming connections.

A client connection factory is used
by an outbound channel adapter but a reference to a client connection factory
can also be provided to an inbound channel adapter and that adapter will receive
any incoming messages received on connections created by the outbound adapter.

A server connection factory is used by an inbound channel adapter or gateway (in fact
the connection factory will not function without one). A reference to a server
connection factory can also be provided to an outbound adapter; that adapter
can then be used to send replies to incoming messages to the same connection.

Tip

Reply messages will only be routed to the connection if the reply contains
the header ip_connection_id that was inserted into the original message by
the connection factory.

Tip

This is the extent of message correlation performed when sharing connection
factories between inbound and outbound adapters. Such sharing allows for
asynchronous two-way communication over TCP. Only payload information is
transferred using TCP; therefore any message correlation must be performed
by downstream components such as aggregators or other endpoints.
For more information refer to Section 16.7, “TCP Message Correlation”.

A maximum of one adapter of each type may be given a reference to a connection
factory.

Connection factories using java.net.Socket and
java.nio.channel.SocketChannel are provided.

A client connection factory that uses java.nio.channel.Socket
connections and creates a new connection for each message.

TCP is a streaming protocol; this means that some structure has to be provided to data
transported over TCP, so the receiver can demarcate the data into discrete messages.
Connection factories are configured to use (de)serializers to convert between the message
payload and the bits that are sent over TCP. This is accomplished by providing a
deserializer and serializer for inbound and outbound messages respectively.
A number of standard (de)serializers are provided.

The ByteArrayCrlfSerializer,
converts a byte array to a stream of bytes followed by carriage
return and linefeed characters (\r\n). This is the default (de)serializer and can be used with
telnet as a client, for example.

The ByteArrayStxEtxSerializer,
converts a byte array to a stream of bytes preceded by an STX (0x02) and
followed by an ETX (0x03).

The ByteArrayLengthHeaderSerializer,
converts a byte array to a stream of bytes preceded by a binary
length in network byte order (big endian). This a very efficient deserializer
because it does not have to parse every byte looking for a termination
character sequence. It can also be used for payloads containing binary data;
the above serializers only support text in the payload. The default size of
the length header is 4 bytes (Integer), allowing for messages up to 2**31-1
bytes. However, the length header can be a single byte (unsigned) for
messages up to 255 bytes, or an unsigned short (2 bytes) for messages up to
2**16 bytes. If you need any other format for the header, you can subclass
this class and provide implementations for the readHeader and writeHeader
methods. The absolute maximum data size supported is 2**31-1 bytes.

The ByteArrayRawSerializer,
converts a byte array to a stream of bytes and adds no additional message
demarcation data; with this (de)serializer, the end of a message is indicated
by the client closing the socket in an orderly fashion. When using this serializer,
message reception will hang until the client closes the socket, or a timeout occurs;
a timeout will NOT result in a message. When this serializer is being used, and the client
is a Spring Integration application, the client must use a connection factory that is
configured with single-use=true - this causes the adapter to close the socket after sending
the message; the serializer will not, itself, close the connection. This serializer
should only be used with connection factories used by channel adapters (not gateways), and the
connection factories should be used by either an inbound or outbound adapter, and not both.

Each of these is a subclass of
AbstractByteArraySerializer which implements both
org.springframework.core.serializer.Serializer and
org.springframework.core.serializer.Deserializer.
For backwards compatibility, connections using any subclass of
AbstractByteArraySerializer for serialization
will also accept a String which will be converted to a byte array first.
Each of these (de)serializers converts an input stream containing the
corresponding format to a byte array payload.

To avoid memory exhaustion due to a badly behaved client (one that does not adhere to
the protocol of the configured serializer), these serializers impose a maximum message
size. If the size is exceeded by an incoming message, an exception will be thrown.
The default maximum message size is 2048 bytes, and can be increased by setting the
maxMessageSize property. If you are using the default (de)serializer
and wish to increase the maximum message size, you must declare it as an explicit bean
with the property set and configure the connection factory to use that bean.

The final standard serializer is
org.springframework.core.serializer.DefaultSerializer which can be
used to convert Serializable objects using java serialization.
org.springframework.core.serializer.DefaultDeserializer is provided for
inbound deserialization of streams containing Serializable objects.

To implement a custom (de)serializer pair, implement the
org.springframework.core.serializer.Deserializer and
org.springframework.core.serializer.Serializer interfaces.

If you do not wish to use
the default (de)serializer (ByteArrayCrLfSerializer), you must supply
serializer and
deserializer attributes on the connection factory (example below).

A server connection factory that uses java.net.Socket
connections and uses Java serialization on the wire.

For full details of the attributes available on connection factories, see the
reference at the end of this section.

By default, reverse DNS lookups are done on inbound packets to convert IP addresses to
hostnames for use in message headers.
In environments where DNS is not configured, this can cause connection delays.
This default behavior can be overridden by setting the lookup-host
attribute to "false".

16.4 Tcp Connection Interceptors

Connection factories can be configured with a reference to a
TcpConnectionInterceptorFactoryChain. Interceptors can be used
to add behavior to connections, such as negotiation, security, and other setup.
No interceptors are currently provided by the framework but, for an example,
see the InterceptedSharedConnectionTests in the source
repository.

The HelloWorldInterceptor used in the test case works as follows:

When configured with a client connection factory,
when the first message is sent over a connection that is intercepted, the interceptor
sends 'Hello' over the connection, and expects to receive 'world!'. When that occurs,
the negotiation is complete and the original message is sent; further messages
that use the same connection are sent without any additional negotiation.

When configured with a server connection factory, the interceptor requires the first
message to be 'Hello' and, if it is, returns 'world!'. Otherwise it throws an exception causing
the connection to be closed.

All TcpConnection methods are intercepted.
Interceptor instances are created for each connection by an interceptor factory.
If an interceptor is stateful, the factory should create a new instance for each connection.
Interceptor
factories are added to the configuration of an interceptor factory chain, which is provided
to a connection factory using the interceptor-factory attribute.
Interceptors must implement the TcpConnectionInterceptor interface;
factories
must implement the TcpConnectionInterceptorFactory interface. A
convenience class AbstractTcpConnectionInterceptor is provided
with passthrough methods; by extending this class, you only need to implement those
methods you wish to intercept.

16.5 TCP Adapters

TCP inbound and outbound channel adapters that utilize the above connection
factories are provided. These adapters have just 2 attributes
connection-factory and channel.
The channel attribute specifies the channel on which messages arrive at an
outbound adapter and on which messages are placed by an inbound adapter.
The connection-factory attribute indicates which connection factory is to be used to
manage connections for the adapter. While both inbound and outbound adapters
can share a connection factory, server connection factories are always 'owned'
by an inbound adapter; client connection factories are always 'owned' by an
outbound adapter. One, and only one, adapter of each type may get a reference
to a connection factory.

In this configuration, messages arriving in channel 'input'
are serialized over connections created by 'client' received
at the server and placed on channel 'loop'. Since 'loop' is
the input channel for 'outboundServer' the message is simply
looped back over the same connection and received by
'inboundClient' and deposited in channel 'replies'. Java
serialization is used on the wire.

16.6 TCP Gateways

The inbound TCP gateway TcpInboundGateway
and outbound TCP gateway TcpOutboundGateway
use a server and client connection factory respectively. Each connection
can process a single request/response at a time.

The inbound gateway, after constructing a message with the incoming payload and sending
it to the requestChannel, waits for a response and sends the payload
from the response message by writing it to the connection.

The outbound gateway, after sending a message over the connection, waits for a response and
constructs a response message and puts in on the reply channel.
Communications over the connections are single-threaded. Users should be aware that only one
message can be handled at a time and, if another thread attempts to send
a message before the current response has been received, it will block until
any previous requests are complete (or time out).
If, however, the client connection factory is configured for single-use connections
each new request gets its own connection and is processed immediately.

A simple inbound TCP gateway; if a connection factory configured with the default
(de)serializer is used, messages will be \r\n delimited data and the gateway can be
used by a simple client such as telnet.

16.7 TCP Message Correlation

16.7.1 Overview

One goal of the IP Endpoints is to provide communication with systems other
than another Spring Integration application. For this reason, only
message payloads are sent
and received. No message correlation is provided by the framework,
except when using the gateways, or collaborating channel adapters on the
server side. In the paragraphs below we discuss the various
correlation techniques available to applications. In most cases, this
requires specific application-level correlation of messages, even when
message payloads contain some natural correlation data (such as an order
number).

16.7.2 Gateways

The gateways will automatically correlate messages. However, an outbound
gateway should only be used for relatively low-volume use.
When the connection factory is configured for a single shared connection
to be used for all message pairs ('single-use="false"'),
only one message can be processed at a time.
A new message will have to wait until the reply to the previous message has
been received.
When a connection factory is configured for each new message to use a new connection
('single-use="true"'), the above restriction does not apply.
While this may give higher throughput
than a shared connection environment, it comes with the overhead of opening
and closing a new connection for each message pair.

Therefore, for high-volume messages, consider using a collaborating pair of
channel adapters. However, you will need to provide collaboration logic.

16.7.3 Collaborating Outbound and Inbound Channel Adapters

To achieve high-volume throughput (avoiding the pitfalls of using gateways
as mentioned above) you may consider configuring a pair of collaborating
outbound and inbound channel adapters. On the server side, message
correlation is automatically handled by the adapters because the inbound
adapter adds a header allowing the outbound adapter to determine which
connection to use to send the reply message. On the client side, however,
the application will have to provide its own correlation logic. This can
be done in a number of ways.

If the message payload has some natural correlation data, such as a
transaction id or an order number, AND there is no need to retain any
information (such as a reply channel header) from the original outbound message,
the correlation is simple and would done at the application level in any case.

If the message payload has some natural correlation data, such as a
transaction id or an order number, but there is a need to retain some
information (such as a reply channel header) from the original outbound message,
you may need to retain a copy of the original outbound message (perhaps
by using a publish-subscribe channel) and use an aggregator to recombine
the necessary data.

For either of the previous two paragraphs, if the payload has no natural
correlation data, you may need to provide a transformer upstream of the
outbound channel adapter to enhance the payload with such data. Such a
transformer may transform the original payload to a new object containing
both the original payload and some subset of the message headers. Of course,
live objects (such as reply channels) from the headers can not be
included in the transformed payload.

If such a strategy is chosen you will need to ensure the connection factory
has an appropriate serializer/deserializer pair to handle such a payload,
such as the DefaultSerializer/Deserializer which use java
serialization, or a custom serializer and deserializer.
The ByteArray*Serializer options
mentioned in Section 16.3, “TCP Connection Factories”,
including the default ByteArrayCrLfSerializer,
do not support such payloads,
unless the transformed payload is a String or
byte[],

16.8 A Note About NIO

Using NIO (see using-nio in
Section 16.9, “IP Configuration Attributes”)
avoids dedicating a thread to read from each socket. For a small number
of sockets, you will likely find that not using NIO,
together with an async handoff (e.g. to a QueueChannel),
will perform as well as, or better than, using NIO.

Consider using NIO when handling a large number of connections. However, the use
of NIO has some other ramifications. A pool of threads (in the task executor) is
shared across all the sockets; each incoming message is assembled and sent to the
configured channel as a separate unit of work on a thread selected from that pool.
Two sequential
messages arriving on the same socket might
be processed by different threads. This means that the order in which the messages are
sent to the channel is indeterminate; the strict ordering of the messages on the
socket is not maintained.

For some applications, this is not an issue; for others it is. If strict ordering
is required, consider setting using-nio to false
and using async handoff.

16.9 IP Configuration Attributes

Table 16.1. Connection Factory Attributes

Attribute Name

Client?

Server?

Allowed Values

Attribute Description

type

Y

Y

client, server

Determines whether the connection factory is a client or server.

host

Y

N

The host name or ip address of the destination.

port

Y

Y

The port.

serializer

Y

Y

An implementation of Serializer used to serialize
the payload. Defaults to ByteArrayCrLfSerializer

deserializer

Y

Y

An implementation of Deserializer used to deserialize
the payload. Defaults to ByteArrayCrLfSerializer

When using NIO, whether or not the connection uses direct buffers.
Refer to java.nio.ByteBuffer documentation for
more information. Must be false if using-nio is false.

so-timeout

Y

Y

See java.net.Socket
setSoTimeout() methods for more information.

so-send-buffer-size

Y

Y

See java.net.Socket
setSendBufferSize() methods for more information.

so-receive-buffer- size

Y

Y

See java.net.Socket
setReceiveBufferSize() methods for more information.

so-keep-alive

Y

Y

true, false

See java.net.Socket. setKeepAlive().

so-linger

Y

Y

Sets linger to true with supplied value.
See java.net.Socket. setSoLinger().

so-tcp-no-delay

Y

Y

true, false

See java.net.Socket. setTcpNoDelay().

so-traffic-class

Y

Y

See java.net.Socket. setTrafficClass().

local-address

N

Y

On a multi-homed system, specifies an IP address
for the interface to which the socket will be bound.

task-executor

Y

Y

Specifies a specific Executor to be used for socket handling. If not supplied, an internal
pooled executor will be used. Needed on some platforms that require the use of specific
task executors such as a WorkManagerTaskExecutor. See pool-size for thread
requirements, depending on other options.

single-use

Y

Y

true, false

Specifies whether a connection can be used for multiple messages.
If true, a new connection will be used for each message.

pool-size

Y

Y

Specifies the concurrency. For tcp, not using nio, specifies the
number of concurrent connections supported by the adapter. For tcp,
using nio, specifies the number of tcp fragments that are concurrently
reassembled into complete messages.
It only applies in this sense if task-executor is not configured.
However, pool-size is also used for the server socket backlog,
regardless of whether an external task executor is used. Defaults to 5.

lookup-host

Y

Y

true, false

Specifies whether reverse lookups are done on IP addresses to convert to host names
for use in message headers. If false, the IP address is used instead. Defaults to true.

When multicast is true, the multicast address to which the adapter
joins.

pool-size

Specifies the concurrency. Specifies how many packets can
be handled concurrently.
It only applies if task-executor is not configured.
Defaults to 5.

task-executor

Specifies a specific Executor to be used for socket handling. If not supplied, an internal
pooled executor will be used. Needed on some platforms that require the use of specific
task executors such as a WorkManagerTaskExecutor. See pool-size for thread
requirements.

receive-buffer-size

The size of the buffer used to receive DatagramPackets.
Usually set to the MTU size. If a smaller buffer is used than the
size of the sent packet, truncation can occur. This can be detected
by means of the check-length attribute..

check-length

true, false

Whether or not a udp adapter expects a data length field in the
packet received. Used to detect packet truncation.

so-timeout

See java.net.DatagramSocket
setSoTimeout() methods for more information.

so-send-buffer-size

Used for udp acknowledgment packets. See java.net.DatagramSocket
setSendBufferSize() methods for more information.

so-receive-buffer- size

See java.net.DatagramSocket
setReceiveBufferSize() for more information.

local-address

On a multi-homed system, specifies an IP address
for the interface to which the socket will be bound.

error-channel

If an Exception is thrown by a downstream
component, the MessagingException message containing the exception
and failed message is sent to this channel.

lookup-host

true, false

Specifies whether reverse lookups are done on IP addresses to convert to host names
for use in message headers. If false, the IP address is used instead. Defaults to true.

Table 16.3. UDP Outbound Channel Adapter Attributes

Attribute Name

Allowed Values

Attribute Description

host

The host name or ip address of the destination. For multicast udp
adapters, the multicast address.

port

The port on the destination.

multicast

true, false

Whether or not the udp adapter uses multicast.

acknowledge

true, false

Whether or not a udp adapter requires an acknowledgment from the destination.
when enabled, requires setting the following 4 attributes.

ack-host

When acknowledge is true, indicates the host or ip address to which the
acknowledgment should be sent. Usually the current host, but may be
different, for example when Network Address Transaction (NAT) is
being used.

ack-port

When acknowledge is true, indicates the port to which the
acknowledgment should be sent. The adapter listens on this port for
acknowledgments.

ack-timeout

When acknowledge is true, indicates the time in milliseconds that the
adapter will wait for an acknowledgment. If an acknowledgment is not
received in time, the adapter will throw an exception.

min-acks-for- success

Defaults to 1. For multicast adapters, you can set this to a larger
value, requiring acknowledgments from multiple destinations.

check-length

true, false

Whether or not a udp adapter includes a data length field in the
packet sent to the destination.

time-to-live

For multicast adapters, specifies the time to live attribute for
the MulticastSocket; controls the scope
of the multicasts. Refer to the Java API
documentation for more information.

so-timeout

See java.net.DatagramSocket
setSoTimeout() methods for more information.

so-send-buffer-size

See java.net.DatagramSocket
setSendBufferSize() methods for more information.

so-receive-buffer- size

Used for udp acknowledgment packets. See java.net.DatagramSocket
setReceiveBufferSize() methods for more information.

local-address

On a multi-homed system, for the UDP adapter, specifies an IP address
for the interface to which the socket will be bound for reply messages.
For a multicast adapter it is also used to determine which interface
the multicast packets will be sent over.

task-executor

Specifies a specific Executor to be used for acknowledgment handling. If not supplied, an internal
single threaded executor will be used. Needed on some platforms that require the use of specific
task executors such as a WorkManagerTaskExecutor. One thread will be dedicated to handling
acknowledgments (if the acknowledge option is true).

Table 16.4. TCP Inbound Channel Adapter Attributes

Attribute Name

Allowed Values

Attribute Description

channel

The channel to which inbound messages will be sent.

connection-factory

If the connection factory has a type 'server', the factory is 'owned'
by this adapter. If it has a type 'client', it is 'owned' by an
outbound channel adapter and this adapter will receive any
incoming messages on the connection created by the
outbound adapter.

error-channel

If an Exception is thrown by a downstream
component, the MessagingException message containing the exception
and failed message is sent to this channel.

Table 16.5. TCP Outbound Channel Adapter Attributes

Attribute Name

Allowed Values

Attribute Description

channel

The channel on which outbound messages arrive.

connection-factory

If the connection factory has a type 'client', the factory is
'owned' by this adapter. If it has a type 'server', it is 'owned'
by an inbound channel adapter and this adapter will attempt
to correlate messages to the connection on which an
original inbound message was received.

Table 16.6. TCP Inbound Gateway Attributes

Attribute Name

Allowed Values

Attribute Description

connection-factory

The connection factory must be of type server.

request-channel

The channel to which incoming messages will be sent.

reply-channel

The channel on which reply messages may arrive. Usually replies will
arrive on a temporary reply channel added to the inbound message
header

reply-timeout

The time in milliseconds for which the gateway will wait for a reply.
Default 1000 (1 second).

error-channel

If an Exception is thrown by a downstream
component, the MessagingException message containing the exception
and failed message is sent to this channel;
any reply from that flow will then be returned as a response by
the gateway.

Table 16.7. TCP Outbound Gateway Attributes

Attribute Name

Allowed Values

Attribute Description

connection-factory

The connection factory must be of type client.

request-channel

The channel on which outgoing messages will arrive.

reply-channel

Optional. The channel to which reply messages may be sent if the
original outbound message did not contain a reply channel header.

reply-timeout

The time in milliseconds for which the gateway will wait for a reply.
Default: 10000 (10 seconds).

request-timeout

If a single-use connection factory is not being used, The time in milliseconds
for which the gateway will wait to get access to the shared connection.

17. JDBC Support

17.1 Inbound Channel Adapter

The main function of an inbound Channel Adapter is to execute a SQL
SELECT query and turn the result set as a message. The
message payload is the whole result set, expressed as a
List, and the types of the items in the list
depend on the row-mapping strategy that is used. The default strategy is
a generic mapper that just returns a Map for each
row in the query result. Optionally, this can be changed by adding a reference to
a RowMapper instance (see the
Spring
JDBC documentation for more detailed information about row mapping).

Note

If you want to convert rows in the SELECT query result to
individual messages you can use a downstream splitter.

The inbound adapter also requires a reference to either
a JdbcTemplate instance or
a DataSource.

As well as the SELECT statement to generate the
messages, the adapter above also has an UPDATE statement that
is being used to mark the records as processed so that they don't show up in
the next poll. The update can be parameterized by the list of ids from the
original select. This is done through a naming convention by default (a
column in the input result set called "id" is translated into a list in
the parameter map for the update called "id"). The following example
defines an inbound Channel Adapter with an update query and a
DataSource reference.

<jdbc:inbound-channel-adapterquery="select * from item where status=2"channel="target"data-source="dataSource"update="update item set status=10 where id in (:id)" />

Note

The parameters in the update query are specified with a colon (:) prefix to the name of a parameter (which in this case is an expression to be applied to each of the rows in the polled result set). This is a standard feature of the named parameter JDBC support in Spring JDBC combined with a convention (projection onto the polled result list) adopted in Spring Integration. The underlying Spring JDBC features limit the available expressions (e.g. most special characters other than period are disallowed), but since the target is usually a list of or an individual object addressable by simple bean paths this isn't unduly restrictive.

To change the parameter generation strategy you can inject a
SqlParameterSourceFactory into the adapter to
override the default behavior (the adapter has a
sql-parameter-source-factory attribute).

17.1.1 Polling and Transactions

The inbound adapter accepts a regular Spring Integration poller as
a sub element, so for instance the frequency of the polling can be
controlled. A very important feature of the poller for JDBC usage is the
option to wrap the poll operation in a transaction, for example:

If a poller is not explicitly specified, a default value will be used (and as per normal with Spring Integration can be defined as a top level bean).

In this example the database is polled every 1000 milliseconds, and the update and
select queries are both executed in the same transaction. The transaction manager
configuration is not shown, but as long as it is aware of the data source then the
poll is transactional. A common use case is for the downstream channels to be
direct channels (the default), so that the endpoints are invoked in the
same thread, and hence the same transaction. Then if any of them fail,
the transaction rolls back and the input data is reverted to its
original state.

17.2 Outbound Channel Adapter

The outbound Channel Adapter is the inverse of the inbound: its role
is to handle a message and use it to execute a SQL query. The message
payload and headers are available by default as input parameters to the
query, for instance:

In the example above, messages arriving on the channel "input" have a payload of
a map with key "foo", so the [] operator dereferences that
value from the map. The headers are also accessed as a map.

Note

The parameters in the query above are bean property expressions on the incoming message (not Spring EL expressions). This behavior is part of the
SqlParameterSource
which is the default source created by the outbound adapter. Other behavior is possible in the adapter, and requires the user to inject a different
SqlParameterSourceFactory.

The outbound adapter requires a reference to either a DataSource or
a JdbcTemplate. It can also have a
SqlParameterSourceFactory injected to control the
binding of incoming message to the query.

If the input channel is a direct channel then the outbound adapter
runs its query in the same thread, and therefore the same transaction (if
there is one) as the sender of the message.

17.3 Outbound Gateway

The outbound Gateway is like a combination of the outbound and
inbound adapters: its role is to handle a message and use it to execute a
SQL query and then respond with the result sending it to a reply channel.
The message payload and headers are available by default as input
parameters to the query, for instance:

The result of the above would be to insert a record into the "foos"
table and return a message to the output channel indicating the number of
rows affected (the payload is a map: {UPDATED=1}).

If the update query is an insert with auto-generated keys, the reply
message can be populated with the generated keys by adding
keys-generated="true" to the above example (this is not
the default because it is not supported by some database platforms). For
example:

As with the channel adapters, there is also the option to provide
SqlParameterSourceFactory instances for request and
reply. The default is the same as for the outbound adapter, so the request
message is available as the root of an expression. If
keys-generated="true" then the root of the expression is the generated
keys (a map if there is only one or a list of maps if
multi-valued).

The outbound gateway requires a reference to either a DataSource or
a JdbcTemplate. It can also have a
SqlParameterSourceFactory injected to control the
binding of the incoming message to the query.

17.4 Message Store

The JDBC module provides an implementation of the Spring Integration
MessageStore (important in the Claim Check pattern)
and MessageGroupStore (important in stateful
patterns like Aggregator) backed by a database. Both interfaces are
implemented by the JdbcMessageStore, and there is also support for
configuring store instances in XML. For example:

Here we have specified a LobHandler for dealing with
messages as large objects (e.g. often necessary if using Oracle) and a
prefix for the table names in the queries generated by the store. The
table name prefix defaults to "INT_".

17.4.1 Initializing the Database

Spring Integration ships with some sample scripts that can be used
to initialize a database. In the spring-integration-jdbc JAR file you
will find scripts in the
org.springframework.integration.jdbc package:
there is a create and a drop script example for a range of common
database platforms. A common way to use these scripts is to reference
them in a Spring
JDBC data source initializer. Note that the scripts are provided
as samples or specifications of the the required table and column names.
You may find that you need to enhance them for production use (e.g. with
index declarations).

17.4.2 Partitioning a Message Store

It is common to use a JdbcMessageStore as a
global store for a group of applications, or nodes in the same
application. To provide some protection against name clashes, and to
give control over the database meta-data configuration, the message
store allows the tables to be partitioned in two ways. One is to use
separate table names, by changing the prefix as described above, and the
other is to specify a "region" name for partitioning data within a
single table. An important use case for this is when the MessageStore is
managing persistent queues backing a Spring Integration Message Channel. The
message data for a persistent channel is keyed in the store on the
channel name, so if the channel names are not globally unique then there
is the danger of channels picking up data that was not intended for
them. To avoid this, the message store region can be used to keep data
separate for different physical channels that happen to have the same
logical name.

18. JMS Support

Spring Integration provides Channel Adapters for receiving and sending JMS messages. There are actually two
JMS-based inbound Channel Adapters. The first uses Spring's JmsTemplate to receive based on
a polling period. The second is "message-driven" and relies upon a Spring MessageListener container. There is also
an outbound Channel Adapter which uses the JmsTemplate to convert and send a JMS Message on
demand.

As you can see from above by using JmsTemplate and MessageListener container Spring Integration relies on Spring's JMS support.
This is important to understand since most of the attributes exposed on these adapters will configure the underlying Spring's JmsTemplate and/or
MessageListener container. For more details about JmsTemplate and MessageListener container please refer to
Spring JMS documentation.

Whereas the JMS Channel Adapters are intended for unidirectional Messaging (send-only or receive-only), Spring
Integration also provides inbound and outbound JMS Gateways for request/reply operations. The inbound gateway
relies on one of Spring's MessageListener container implementations for Message-driven reception that is also
capable of sending a return value to the "reply-to" Destination as provided by the received Message. The outbound
Gateway sends a JMS Message to a "request-destination" and then receives a reply Message. The "reply-destination"
reference (or "reply-destination-name") can be configured explicitly or else the outbound gateway will use a
JMS TemporaryQueue.

18.1 Inbound Channel Adapter

The inbound Channel Adapter requires a reference to either a single JmsTemplate
instance or both ConnectionFactory and Destination
(a 'destinationName' can be provided in place of the 'destination' reference). The following example defines an
inbound Channel Adapter with a Destination reference.

Notice from the configuration that the inbound-channel-adapter is a Polling Consumer. That means that
it invokes receive() when triggered. This should only be used in situations where polling is done relatively
infrequently and timeliness is not important. For all other situations (a vast majority of JMS-based use-cases),
the message-driven-channel-adapter described below is a better option.

Note

All of the JMS adapters that require a reference to the ConnectionFactory will automatically look for
a bean named "connectionFactory" by default. That is why you don't see a "connection-factory" attribute
in many of the examples. However, if your JMS ConnectionFactory has a different bean name, then you will
need to provide that attribute.

If 'extract-payload' is set to true (which is the default), the received JMS Message will be passed through
the MessageConverter. When relying on the default SimpleMessageConverter, this means that the resulting Spring
Integration Message will have the JMS Message's body as its payload. A JMS TextMessage will produce a
String-based payload, a JMS BytesMessage will produce a byte array payload, and a JMS ObjectMessage's
Serializable instance will become the Spring Integration Message's payload. If instead you prefer to have
the raw JMS Message as the Spring Integration Message's payload, then set 'extract-payload' to false.

18.2 Message-Driven Channel Adapter

The "message-driven-channel-adapter" requires a reference to either an instance of a Spring MessageListener
container (any subclass of AbstractMessageListenerContainer) or both
ConnectionFactory and Destination
(a 'destinationName' can be provided in place of the 'destination' reference). The following example defines a
message-driven Channel Adapter with a Destination reference.

The Message-Driven adapter also accepts several properties that pertain to the MessageListener container.
These values are only considered if you do not provide an actual 'container' reference. In that case,
an instance of DefaultMessageListenerContainer will be created and configured based on these properties.
For example, you can specify the "transaction-manager" reference, the "concurrent-consumers" value, and
several other property references and values. Refer to the JavaDoc and Spring Integration's JMS Schema
(spring-integration-jms-2.0.xsd) for more detail.

The 'extract-payload' property has the same effect as described above, and once again its default value
is 'true'. The poller sub-element is not applicable for a message-driven
Channel Adapter, as it will be actively invoked. For most usage scenarios, the message-driven approach is better since the Messages will
be passed along to the MessageChannel as soon as they are received from the underlying
JMS consumer.

When comparing this to the generic gateway configuration, or the JMS 'inbound-gateway' that will
be discussed below, the key difference here is that we are in a one-way flow
since this is a 'channel-adapter', not a gateway. Therefore, the flow downstream from the
'error-channel' should also be one-way. For example, it could simply send to a logging handler,
or it could be connected to a different JMS <outbound-channel-adapter> element.

18.3 Outbound Channel Adapter

The JmsSendingMessageHandler implements the MessageHandler
interface and is capable of converting Spring Integration Messages to JMS messages
and then sending to a JMS destination. It requires either a 'jmsTemplate' reference or both 'connectionFactory' and
'destination' references (again, the 'destinationName' may be provided in place of the 'destination'). As with the
inbound Channel Adapter, the easiest way to configure this adapter is with the namespace support. The following
configuration will produce an adapter that receives Spring Integration Messages from the "exampleChannel" and then
converts those into JMS Messages and sends them to the JMS Destination reference whose bean name is "outQueue".

As with the inbound Channel Adapters, there is an 'extract-payload' property. However, the meaning is reversed
for the outbound adapter. Rather than applying to the JMS Message, the boolean property applies to the Spring
Integration Message payload. In other words, the decision is whether to pass the Spring Integration Message
itself as the JMS Message body or whether to pass the Spring Integration Message's
payload as the JMS Message body. The default value is once again 'true'. Therefore, if you pass a Spring
Integration Message whose payload is a String, a JMS TextMessage will be created. If on the other hand you
want to send the actual Spring Integration Message to another system via JMS, then simply set this to 'false'.

Note

Regardless of the boolean value for payload extraction, the Spring Integration MessageHeaders will map to
JMS properties as long as you are relying on the default converter or provide a reference to another
instance of HeaderMappingMessageConverter (the same holds true for 'inbound' adapters except that in
those cases, it's the JMS properties mapping to Spring Integration MessageHeaders).

18.4 Inbound Gateway

Spring Integration's message-driven JMS inbound-gateway delegates to a
MessageListener container, supports dynamically adjusting concurrent consumers,
and can also handle replies. The inbound gateway requires references to a
ConnectionFactory, and a request Destination (or
'requestDestinationName'). The following example defines a JMS "inbound-gateway" that receives from the JMS
queue referenced by the bean id "inQueue" and sends to the Spring Integration channel named "exampleChannel".

Since the gateways provide request/reply behavior instead of unidirectional send or
receive, they also have two distinct properties for the "payload extraction" (as discussed above for the
Channel Adapters' 'extract-payload' setting). For an inbound-gateway, the 'extract-request-payload' property
determines whether the received JMS Message body will be extracted. If 'false', the JMS Message itself will
become the Spring Integration Message payload. The default is 'true'.

Similarly, for an inbound-gateway the 'extract-reply-payload' property applies to the Spring Integration Message
that is going to be converted into a reply JMS Message. If you want to pass the whole Spring Integration Message
(as the body of a JMS ObjectMessage) then set this to 'false'. By default, it is also 'true' such that the Spring
Integration Message payload will be converted into a JMS Message (e.g. String payload
becomes a JMS TextMessage).

As with anything else, Gateway invocation might result in error.
By default Producer will not be notified of the errors that might have occurred on the consumer side and will time out waiting for
the reply. However there might be times when you want to communicate an error condition back to the consumer,
in other words treat the Exception as a valid reply by mapping it to a Message. To accomplish this
JMS Inbound Gateway provides support for a Message Channel to which errors
can be sent for processing, potentially resulting in a reply Message payload
that conforms to some contract defining what a caller may expect as an "error"
reply. Such a channel can be configured via the error-channel
attribute.

You might notice that this example looks very similar to that included
within Section 7.2.1, “GatewayProxyFactoryBean”.
The same idea applies here: The exceptionTransformer
could be a simple POJO that creates error response objects, you could reference
the "nullChannel" to suppress the errors, or you could leave 'error-channel' out
to let the Exception propagate.

18.5 Outbound Gateway

The outbound Gateway creates JMS Messages from Spring Integration Messages and then sends to a
'request-destination'. It will then handle the JMS reply Message either by using a selector to
receive from the 'reply-destination' that you configure, or if no 'reply-destination' is provided,
it will create JMS TemporaryQueues. Notice that the "reply-channel" is also provided.

The 'outbound-gateway' payload extraction properties are inversely related to those of the
'inbound-gateway' (see the discussion above). That means that the 'extract-request-payload' property value
applies to the Spring Integration Message that is being converted into a JMS Message to be
sent as a request, and the 'extract-reply-payload' property value applies to the
JMS Message that is received as a reply and then converted into a Spring Integration
Message to be subsequently sent to the 'reply-channel' as shown in the example configuration above.

18.6 Mapping Message Headers to/from JMS Message

JMS Message can contain meta-information such as JMS API headers as well as simple properties.
You can map those to/from Spring Integration Message Headers using JmsHeaderMapper.
The JMS API headers are passed to the appropriate setter methods (e.g. setJMSReplyTo) whereas other headers will
be copied to the general properties of the JMS Message.
JMS Outbound Gateway is bootstrapped with the default implementation of JmsHeaderMapper which will map
standard JMS API Headers as well as primitive/String Message Headers. Custom header mapper could also be
provided via header-mapper attribute of inbound and outbound gateways.

18.7 Message Conversion, Marshalling and Unmarshalling

If you need to convert the message, all JMS adapters and gateways, allow you to
provide a MessageConverter via message-converter attribute. Simply provide the
bean name of an instance of MessageConverter that is available within the same
ApplicationContext.
Also, to provide some consistency with Marshaller and Unmarshaller interfaces Spring provides MarshallingMessageConverter
which you can configure with your own custom Marshallers and Unmarshallers

Note, however, that when you provide your own MessageConverter instance, it will still
be wrapped within the HeaderMappingMessageConverter. This means that the 'extract-request-payload'
and 'extract-reply-payload' properties may affect what actual objects are passed to your converter. The
HeaderMappingMessageConverter itself simply delegates to a target MessageConverter while also mapping the
Spring Integration MessageHeaders to JMS Message properties and vice-versa.

18.8 JMS Backed Message Channels

The Channel Adapters and Gateways featured above are all intended for applications that are integrating
with other external systems. The inbound options assume that some other system is sending JMS Messages
to the JMS Destination and the outbound options assume that some other system is receiving from the
Destination. The other system may or may not be a Spring Integration application. Of course, when sending
the Spring Integration Message instance as the body of the JMS Message itself (with the 'extract-payload'
value set to false), it is assumed that the other system is based on Spring Integration. However,
that is by no means a requirement. That flexibility is one of the benefits of using a Message-based
integration option with the abstraction of "channels" or Destinations in the case of JMS.

There are cases where both the producer and consumer for a given JMS Destination are intended to be
part of the same application, running within the same process. This could be accomplished by using a
pair of inbound and outbound Channel Adapters. The problem with that approach is that two adapters are
required even though conceptually the goal is to have a single Message Channel. A better option is
supported as of Spring Integration version 2.0. Now it is possible to define a single "channel" when
using the JMS namespace.

<jms:channelid="jmsChannel"queue="exampleQueue"/>

The channel in the above example will behave much like a normal <channel/> element from the main
Spring Integration namespace. It can be referenced by both "input-channel" and "output-channel" attributes
of any endpoint. The difference is that this channel is backed by a JMS Queue instance named "exampleQueue".
This means that asynchronous messaging is possible between the producing and consuming endpoints, but
unlike the simpler asynchronous Message Channels created by adding a <queue/> sub-element within a
non-JMS <channel/> element, the Messages are not just stored in an in-memory queue. Instead those
Messages are passed within a JMS Message body, and the full power of the underlying JMS provider is then
available for that channel. Probably the most common rationale for using this alternative would be to
take advantage of the persistence made available by the store and forward approach
of JMS messaging. If configured properly, the JMS-backed Message Channel also supports transactions.
In other words, a producer would not actually write to a transactional JMS-backed channel if its send
operation is part of a transaction that rolls back. Likewise, a consumer would not physically remove a
JMS Message from the channel if the reception of that Message is part of a transaction that rolls back.
Note that the producer and consumer transactions are separate in such a scenario. This is significantly
different than the propagation of a transactional context across the simple, synchronous <channel/>
element that has no <queue/> sub-element.

Since the example above is referencing a JMS Queue instance, it will act as a point-to-point channel. If
on the other hand, publish/subscribe behavior is needed, then a separate element can be used, and a JMS
Topic can be referenced instead.

<jms:publish-subscribe-channelid="jmsChannel"topic="exampleTopic"/>

For either type of JMS-backed channel, the name of the destination may be provided instead of a reference.

In the examples above, the Destination names would be resolved by Spring's default
DynamicDestinationResolver implementation, but any implementation of the
DestinationResolver interface could be provided. Also, the JMS
ConnectionFactory is a required property of the channel, but by default
the expected bean name would be "connectionFactory". The example below provides both a custom instance
for resolution of the JMS Destination names and a different name for the ConnectionFactory.

18.9 JMS Samples

To experiment with these JMS adapters, check out JMS samples available in our new Samples Git repository
available here: http://git.springsource.org/+spring-integration/spring-integration/samples .
There are two samples included. One provides inbound and outbound Channel Adapters, and the
other provides inbound and outbound Gateways. They are configured to run with an embedded ActiveMQ process, but
the "common.xml" file can easily be modified to support either a different JMS provider or a standalone
ActiveMQ process. In other words, you can split the configuration so that the inbound and outbound adapters are
running in separate JVMs. If you have ActiveMQ installed, simply modify the "brokerURL" property within the
configuration to use "tcp://localhost:61616" for example (instead of "vm://localhost"). Both of the samples
accept input via stdin and then echo back to stdout. Look at the configuration to see how these messages are
routed over JMS.

19. RMI Support

19.1 Introduction

This Chapter explains how to use RMI specific channel adapters to distribute a system over multiple JVMs. The first section will deal with sending messages over RMI. The second section shows how to receive messages over RMI. The last section shows how to define rmi channel adapters through the namespace support.

19.2 Outbound RMI

To send messages from a channel over RMI, simply define an RmiOutboundGateway. This gateway will use Spring's RmiProxyFactoryBean internally to create a proxy for a remote gateway. Note that to invoke a remote interface that doesn't use Spring Integration you should use a service activator in combination with Spring's RmiProxyFactoryBean.

20. SFTP Adapters

20.1 Introduction

The Secure File Transfer Protocol (SFTP) is a network protocol which allows you to transfer
files between two computers on the Internet over any reliable stream.

The SFTP protocol requires a secure channel, such as SSH, as well as visibility to a client's identity throughout the SFTP session.

Spring Integration supports sending and receiving files over SFTP by providing two types of clients -
Inbound Channel Adapters and Outbound Channel Adapters as well as convenient
namespace configuration to define these clients.

Every time an adapter requests a session object from its SessionFactory the session is
returned from a session pool maintained by a caching wrapper around the factory. A Session in the session pool might go stale
(if it has been disconnected by the server due to inactivity) so the SessionFactory
will perform validation to make sure that it never returns a stale session to the adapter. If a stale session was encountered,
it will be removed from the pool, and a new one will be created.

Note

If you experience connectivity problems and would like to trace Session creation as well as see which Sessions are
polled you may enable it by setting the logger to TRACE level (e.g., log4j.category.org.springframework.integration.file=TRACE)

Now all you need to do is inject this SFTP Session Factory into your adapters.

Note

A more practical way to provide values for the SFTP Session Factory would be via Spring's property
placeholder support (http://static.springsource.org/spring/docs/3.0.x/spring-framework-reference/html/beans.html#beans-factory-placeholderconfigurer)

20.3 SFTP Inbound Channel Adapter

The SFTP Inbound Channel Adapter is a special listener that will connect to the server and listen for
the remote directory events (e.g., new file created) at which point it will initiate a file transfer.

As you can see from the configuration above you can configure the SFTP Inbound Channel Adapter via the
inbound-channel-adapter element while also providing values for various attributes such as local-directory
- where files are going to be transferred TO and remote-directory - the remote source directory where files are
going to be transferred FROM -
as well as other attributes including a session-factory reference to the bean we configured earlier.

Some times file filtering based on the simple pattern specified via filename-pattern attribute might not be
sufficient. If this is the case, you can use the filename-regex attribute to specify a Regular Expression
(e.g. filename-regex=".*\.test$"). And of course if you need complete control you can use the filter
attribute to provide a reference to a custom implementation of the
org.springframework.integration.file.filters.FileListFilter - a strategy interface for filtering a
list of files.

Please refer to the schema for more detail on these attributes.

It is also important to understand that SFTP Inbound Channel Adapter is a Polling Consumer and therefore
you must configure a poller (either a global default or a local sub-element).
Once the file has been transferred to a local directory, a Message with java.io.File as its
payload type will be generated and sent to the channel identified by the channel attribute.

More on File Filtering and Large Files

Some times a file that just appeared in the monitored (remote) directory is not complete. Typically such a file
will be written with some temporary extension (e.g., foo.txt.writing) and then renamed after the writing process completes.
As a user in most cases you are only interested in files that are complete and would like to filter only those files.
To handle these scenarios, use filtering support provided via the filename-pattern, filename-regex
and filter attributes.
If you need a custom filter implementation simply include a reference in your adapter via the filter attribute.

20.4 SFTP Outbound Channel Adapter

The SFTP Outbound Channel Adapteris a special MessageHandler that will connect to the
remote directory and will initiate a file transfer for every file it will receive as the payload of an incoming Message.
It also supports several representations of the File so you are not limited to the File object.
Similar to the FTP outbound adapter, the SFTP Outbound Channel Adapter supports the following payloads:
1) java.io.File - the actual file object; 2) byte[] - byte array that represents
the file contents; 3) java.lang.String - text that represents the file contents.

As you can see from the configuration above you can configure the SFTP Outbound Channel Adapter via
the outbound-channel-adapter element.
Please refer to the schema for more detail on these attributes.

SpEL and the SFTP Outbound Adapter

As with many other components in Spring Integration, you can benefit from the Spring Expression Language (SpEL) support when configuring
an SFTP Outbound Channel Adapter, by specifying two attributes remote-directory-expression and
remote-filename-generator-expression (see above). The expression evaluation context will have the Message as its root object, thus allowing
you to provide expressions which can dynamically compute the file name or the existing directory path
based on the data in the Message (either from 'payload' or 'headers'). In the example above we are defining
the remote-filename-generator-expression attribute with an expression
value that computes the file name based on its original name while also appending a suffix: '-foo'.

20.5 SFTP/JSCH Logging

Since we use JSch libraries (http://www.jcraft.com/jsch/) to provide SFTP support, at times you may require
more information from the JSch API itself, especially if something is not working properly
(e.g., Authentication exceptions). Unfortunately JSch does not use commons-logging but instead
relies on custom implementations of their com.jcraft.jsch.Logger interface.
As of Spring Integration 2.0.1, we have implemented this interface. So, now all you need to do to enable
JSch logging is to configure your logger the way you usually do. For example, here is valid configuration of a
logger using Log4J.

log4j.category.com.jcraft.jsch=DEBUG

20.6 SFTP Session Caching

One of the optimizations implemented by the SFTP adapters is session caching. Similar to JDBC pooling of Connections, the SFTP Adapters maintain a
pool of Sessions by default. However there are times when this behavior is not desired (e.g., security etc.).
To disable session caching you can set the cache-sessions attribute to false (the default value is true).

21. Stream Support

21.1 Introduction

In many cases application data is obtained from a stream. It is not recommended to send a reference to a Stream as a message payload to a consumer. Instead messages are created from data that is read from an input stream and message payloads are written to an output stream one by one.

21.2 Reading from streams

Spring Integration provides two adapters for streams. Both ByteStreamReadingMessageSource and
CharacterStreamReadingMessageSource implement MessageSource.
By configuring one of these within a channel-adapter element, the polling period can be configured,
and the Message Bus can automatically detect and schedule them. The byte stream version requires an
InputStream, and the character stream version requires a Reader as
the single constructor argument. The ByteStreamReadingMessageSource also accepts the 'bytesPerMessage'
property to determine how many bytes it will attempt to read into each Message. The
default value is 1024

21.3 Writing to streams

For target streams, there are also two implementations: ByteStreamWritingMessageHandler and
CharacterStreamWritingMessageHandler. Each requires a single constructor argument -
OutputStream for byte streams or Writer for character streams,
and each provides a second constructor that adds the optional 'bufferSize'. Since both of these
ultimately implement the MessageHandler interface, they can be referenced from a
channel-adapter configuration as described in more detail in
Section 3.2, “Channel Adapter”.

22. Twitter Adapter

Spring Integration provides support for interacting with Twitter. With the Twitter adapters you can both
receive and send Twitter messages. You can also perform a Twitter search based on a schedule and publish
the search results within Messages.

22.1 Introduction

Twitter is a social networking and micro-blogging service that enables its users to send and read messages known as tweets.
Tweets are text-based posts of up to 140 characters displayed on the author's profile page and delivered to the author's
subscribers who are known as followers.

Important

Current Twitter support is based on the Twitter4J API.
However, future versions will be changed to use the Spring Social
project as it is nearing its first release at the time of writing.

Spring Integration provides a convenient namespace configuration to define Twitter artifacts. You can enable it by adding
the following within your XML header.

22.2 Twitter OAuth Configuration

The Twitter API allows for both authenticated and anonymous operations. For authenticated operations Twitter uses OAuth
- an authentication protocol that allows users to approve an application to act on their behalf without
sharing their password. More information can be found at http://oauth.net/ or
in this article http://hueniverse.com/oauth/ from Hueniverse.
Please also see OAuth FAQ for more information about OAuth and Twitter.

In order to use OAuth authentication/authorization with Twitter you must create a new Application on the Twitter Developers site.
Follow the directions below to create a new application and obtain consumer keys and an access token:

Click on the Register an app link and fill out all required fields on the form provided;
set Application Type to Client and depending on the nature of your application select
Default Access Type as Read & Write or Read-only
and Submit the form. If everything is successful you'll be presented with the Consumer Key
and Consumer Secret. Copy both values in a safe place.

On the same page you should see a My Access Token button on the side bar (right).
Click on it and you'll be presented with two more values: Access Token and Access Token Secret.
Copy these values in a safe place as well.

22.3 Twitter Template

Spring Integration uses the same familiar template pattern to interact with Twitter. Since current Twitter support
is based on the Twitter4J API we provide a simple Twiter4JTemplate.
For anonymous operations (e.g., search), you don't have to define Twitter4JTemplate explicitly, since a default
instance will be created and injected into the endpoint. However, for authenticated operation
(update status, send direct message, etc.), you must configure Twitter4JTemplate as a bean and
inject it explicitly into the endpoint, because the authentication configuration is required.
Below is a sample configuration of Twitter4JTemplate:

As you can see from the configuration above, all we need to do is to provide
OAuth attributes as constructor arguments. The values would be those you obtained in the previous step.
The order of constructor arguments is: 1) consumerKey, 2) consumerSecret,
3) accessToken, and 4) accessTokenSecret.

A more practical way to manage OAuth connection attributes would be via Spring's property placeholder support by simply
creating a property file (e.g., oauth.properties):

22.4 Twitter Inbound Adapters

The current release of Spring Integration provides support for receiving tweets as Timeline Updates,
Direct Messages, Mention Messages as well as Search Results.

Every Inbound Twitter Channel Adapter is a Polling Consumer which means you have to provide a poller
configuration. However, there is one important thing you must understand about Twitter since its inner-workings are slightly
different than other polling consumers. Twitter defines a concept of Rate Limiting. You can read more about
it here: Rate Limiting. In a nutshell, Rate Limiting
is the way Twitter manages how often an application can poll for updates. You should consider this when setting your
poller intervals, but we are also doing a few things to limit excessively aggressive polling within our adapters.

Another issue that we need to worry about is handling duplicate Tweets. The same adapter (e.g., Search or Timeline Update)
while polling on Twitter may receive the same values more than once. For example if you keep searching on Twitter with the same search
criteria you'll end up with the same set of tweets unless some other new tweet that matches your search criteria was posted
in between your searches. In that situation you'll get all the tweets you had before plus the new one. But what you really
want is only the new tweet(s). Spring Integration provides an elegant mechanism for handling these situations.
The latest Tweet timestamp will be stored in an instance of the org.springframework.integration.store.MetadataStore which is a
strategy interface designed for storing various types of metadata (e.g., last retrieved tweet in this case). That strategy helps components such as
these Twitter adapters avoid duplicates. By default, Spring Integration will look for a bean of type
org.springframework.integration.store.MetadataStore in the ApplicationContext.
If one is found then it will be used, otherwise it will create a new instance of SimpleMetadataStore
which is a simple in-memory implementation that will only persist metadata within the lifecycle of the currently running application context.
That means upon restart you may end up with duplicate entries. If you need to persist metadata between Application Context
restarts, you may use the PropertiesPersistingMetadataStore (which is backed by a properties file, and a persister
strategy), or you may create your own custom implementation of the MetadataStore interface (e.g., JdbcMetadatStore)
and configure it as a bean named 'metadataStore' within the Application Context.

Here is a link that will help you learn more about Twitter queries: http://search.twitter.com/operators

As you can see the configuration of all of these adapters is very similar to other inbound adapters with one exception.
Some may need to be injected with the twitter-template. Once received each Twitter Message would be
encapsulated in a Spring Integration Message and sent to the channel specified by the channel attribute.
Currently the Payload type of any Message is org.springframework.integration.twitter.core.Tweet
which is very similar to the object with the same name in Spring Social. As we migrate to Spring Social
we'll be depending on their API and some of the artifacts that are currently in use will be obsolete, however we've already
made sure that the impact of such migration is minimal by aligning our API with the current state (at the time of writing)
of Spring Social.

To get the text from the org.springframework.integration.twitter.core.Tweet
simply invoke the getText() method.

22.5 Twitter Outbound Adapter

The current release of Spring Integration supports sending Status Update Messages and Direct Messages.
Twitter outbound channel adapters will take the Message payload and send it as a Twitter message. Currently the only supported payload type is
String, so consider adding a transformer if the payload of the incoming message is not a String.

22.5.1 Twitter Outbound Update Channel Adapter

This adapter allows you to send regular status updates by simply sending a Message to the channel
identified by the channel attribute.

The only extra configuration that is required for this adapter is the twitter-template reference.

When it comes to Twitter Direct Messages, you must specify who you are sending the message to - the target userid.
The Twitter Outbound Direct Message Channel Adapter will look for a target userid in the Message headers under the name
twitter_dmTargetUserId which is also identified by the following constant: TwitterHeaders.DM_TARGET_USER_ID.
So when creating a Message all you need to do is add a value for that header.

The above approach works well if you are creating the Message programmatically. However it's more common to
provide the header value within a messaging flow. The value can be provided by an upstream <header-enricher>.

Twitter does not allow you to post duplicate Messages. This is a common problem during testing when
the same code works the first time but does not work the second time. So, make sure to change the content of the Message
each time. Another thing that works well for testing is to append a timestamp to the end of each message.

23. Web Services Support

23.1 Outbound Web Service Gateways

To invoke a Web Service upon sending a message to a channel, there are two options - both of which build
upon the Spring Web Services
project: SimpleWebServiceOutboundGateway and
MarshallingWebServiceOutboundGateway. The former will accept either a
String or javax.xml.transform.Source as the message
payload. The latter provides support for any implementation of the Marshaller
and Unmarshaller interfaces. Both require a Spring Web Services
DestinationProvider for determining the URI of the Web Service to be
called.

When using the namespace support described below, you will only need to set a URI. Internally, the parser
will configure a fixed URI DestinationProvider implementation. If you do need dynamic resolution of the
URI at runtime, however, then the DestinationProvider can provide such behavior as looking up the URI from
a registry. See the Spring Web Services
javadoc for
more information about the DestinationProvider strategy.

For more detail on the inner workings, see the Spring Web Services reference guide's chapter covering
client access
as well as the chapter covering
Object/XML mapping.

23.2 Inbound Web Service Gateways

To send a message to a channel upon receiving a Web Service invocation, there are two options again: SimpleWebServiceInboundGateway and
MarshallingWebServiceInboundGateway. The former will extract a javax.xml.transform.Source
from the WebServiceMessage and set it as the message
payload. The latter provides support for implementation of the Marshaller
and Unmarshaller interfaces.
If the incoming web service message is a SOAP message the SOAP Action header will be added to the headers of the
Message that is forwarded onto the request channel.

Notice that this example does not provide a 'reply-channel'. If the Web Service were to
return a non-empty response, the Message containing that response would be sent to the
reply channel provided in the request Message's REPLY_CHANNEL header, and if that were
not available a channel resolution Exception would be thrown. If you want to send the
reply to another channel instead, then provide a 'reply-channel' attribute on the
'outbound-gateway' element.

Tip

When invoking a Web Service that returns an empty response after using a String payload
for the request Message, no reply Message will be sent by default.
Therefore you don't need to set a 'reply-channel' or have a REPLY_CHANNEL header in the
request Message. If for any reason you actually do want to receive
the empty response as a Message, then provide the 'ignore-empty-responses' attribute with
a value of false (this only applies for Strings, because using a
Source or Document object simply leads to a NULL response and will therefore
never generate a reply Message).

Most Marshaller implementations also implement the
Unmarshaller interface. When using such a
Marshaller, only the "marshaller"
attribute is necessary. Even when using a Marshaller,
you may also provide a reference for the "request-callback" on the outbound gateways.

For either outbound gateway type, a "destination-provider" attribute can be specified instead of the "uri"
(exactly one of them is required). You can then reference any Spring Web Services DestinationProvider
implementation (e.g. to lookup the URI at runtime from a registry).

For either outbound gateway type, the "message-factory" attribute can also be configured with a reference to any
Spring Web Services WebServiceMessageFactory implementation.

For the simple inbound gateway type, the "extract-payload" attribute can be set to false to forward
the entire WebServiceMessage instead of just its payload as a
Message to the request channel. This might be useful, for example,
when a custom Transformer works against the WebServiceMessage directly.

24. XML Support - Dealing with XML Payloads

24.1 Introduction

Spring Integration's XML support extends the Spring Integration Core with
implementations of splitter, transformer, selector and router designed
to make working with xml messages in Spring Integration simple. The provided messaging
components are designed to work with xml represented in a range of formats including
instances of
java.lang.String, org.w3c.dom.Document
and javax.xml.transform.Source. It should be noted however that
where a DOM representation is required, for example in order to evaluate an XPath expression,
the String payload will be converted into the required type and then
converted back again to String. Components that require an instance of
DocumentBuilder will create a namespace aware instance if one is
not provided. Where greater control of the document being created is required an appropriately
configured instance of DocumentBuilder should be provided.

24.2 Transforming xml payloads

This section will explain the workings of
UnmarshallingTransformer,
MarshallingTransformer,
XsltPayloadTransformer
and how to configure them as
beans. All of the provided xml transformers extend
AbstractTransformer or AbstractPayloadTransformer
and therefore implement Transformer. When configuring xml
transformers as beans in Spring Integration you would normally configure the transformer
in conjunction with either a MessageTransformingChannelInterceptor or a
MessageTransformingHandler. This allows the transformer to be used as either an interceptor,
which transforms the message as it is sent or received to the channel, or as an endpoint. Finally the
namespace support will be discussed which allows for the simple configuration of the transformers as
elements in XML.

UnmarshallingTransformer allows an xml Source
to be unmarshalled using implementations of Spring OXM Unmarshaller.
Spring OXM provides several implementations supporting marshalling and unmarshalling using JAXB,
Castor and JiBX amongst others. Since the unmarshaller requires an instance of
Source where the message payload is not currently an instance of
Source, conversion will be attempted. Currently String
and org.w3c.dom.Document payloads are supported. Custom conversion to a
Source is also supported by injecting an implementation of
SourceFactory.

The MarshallingTransformer allows an object graph to be converted
into xml using a Spring OXM Marshaller. By default the
MarshallingTransformer will return a DomResult.
However the type of result can be controlled by configuring an alternative ResultFactory
such as StringResultFactory. In many cases it will be more convenient to transform
the payload into an alternative xml format. To achieve this configure a
ResultTransformer. Two implementations are provided, one which converts to
String and another which converts to Document.

By default, the MarshallingTransformer will pass the payload Object
to the Marshaller, but if its boolean extractPayload property
is set to false, the entire Message instance will be passed
to the Marshaller instead. That may be useful for certain custom
implementations of the Marshaller interface, but typically the
payload is the appropriate source Object for marshalling when delegating to any of the various
out-of-the-box Marshaller implementations.

XsltPayloadTransformer transforms xml payloads using xsl.
The transformer requires an instance of either Resource or
Templates. Passing in a Templates instance
allows for greater configuration of the TransformerFactory used to create
the template instance. As in the case of XmlPayloadMarshallingTransformer
by default XsltPayloadTransformer will create a message with a
Result payload. This can be customised by providing a
ResultFactory and/or a ResultTransformer.

24.3 Namespace support for xml transformers

Namespace support for all xml transformers is provided in the Spring Integration xml namespace,
a template for which can be seen below. The namespace support for transformers creates an instance of either
EventDrivenConsumer or PollingConsumer
according to the type of the provided input channel. The namespace support is designed
to reduce the amount of xml configuration by allowing the creation of an endpoint and transformer
using one element.

The namespace support for UnmarshallingTransformer is shown below.
Since the namespace is now creating an endpoint instance rather than a transformer,
a poller can also be nested within the element to control the polling of the input channel.

The namespace support for the marshalling transformer requires an input channel, output channel and a
reference to a marshaller. The optional result-type attribute can be used to control the type of result created,
valid values are StringResult or DomResult (the default). Where the provided result types are not sufficient a
reference to a custom implementation of ResultFactory can be provided as an alternative
to setting the result-type attribute using the result-factory attribute. An optional result-transformer can also be
specified in order to convert the created Result after marshalling.

Namespace support for the XsltPayloadTransformer allows either a resource to be passed in in order to create the
Templates instance or alternatively a precreated Templates
instance can be passed in as a reference. In common with the marshalling transformer the type of the result output can
be controlled by specifying either the result-factory or result-type attribute. A result-transfomer attribute can also
be used to reference an implementation of ResultTransfomer where conversion of the result
is required before sending.

Very often to assist with transformation you may need to have access to Message data (e.g., Message Headers). For example; you may need to get access to certain Message Headers
and pass them on as parameters to a transformer (e.g., transformer.setParameter(..)).
Spring Integration provides two convenient ways to accomplish this. Just look at the following XML snippet.

If message header names match 1:1 to parameter names, you can simply use xslt-param-headers attribute. There you can also use wildcards for
simple pattern matching which supports the following simple pattern styles: "xxx*", "*xxx", "*xxx*" and "xxx*yyy".

You can also configure individual xslt parameters via <xslt-param/> sub element. There you can use expression or value attribute.
The expression attribute should be any valid SpEL expression with Message being the root object of the expression evaluation context.
The value attribute just like any value in Spring beans allows you to specify simple scalar vallue. YOu can also use property placeholders (e.g., ${some.value})
So as you can see, with the expression and value attribute xslt parameters could now be mapped to any accessible part of the Message as well as any literal value.

24.4 Splitting xml messages

XPathMessageSplitter supports messages with either
String or Document payloads.
The splitter uses the provided XPath expression to split the payload into a number of
nodes. By default this will result in each Node instance
becoming the payload of a new message. Where it is preferred that each message be a Document
the createDocuments flag can be set. Where a String payload is passed
in the payload will be converted then split before being converted back to a number of String
messages. The XPath splitter implements MessageHandler and should
therefore be configured in conjunction with an appropriate endpoint (see the namespace support below
for a simpler configuration alternative).

24.5 Routing xml messages using XPath

Similar to SpEL-based routers, Spring Integration provides support for routing messages based on the XPath expressions
allowing you to create a Message Endpoint with an input channel but no output channel
since the output channel(s) is determined dynamically.

Internally XPath expression will be evaluated as NODESET type and converted to a
List<String> representing channel names. Typically such list will contain a single channel name.
However, based on the result of an XPath Expression XPath router can also take on the characteristics of the Recipient List Router
if XPath Expression returns more then one value, thus resulting in the List<String> containing more
then one channel name. In that case Message will be sent to all channels in the list.
So assuming that the xml file passed to the router configured below contains many responder sub-elements representing channel names,
the message will be sent to all of those channels.

<!-- route the order to all responders--><si-xml:xpath-routerid="responderRouter"input-channel="orderChannel"><si-xml:xpath-expressionexpression="/request/responders"/></si-xml:xpath-router>

If the returned values do not represent the channel names additional mapping could be specified. For example if
the /request/responders expression results in two values responderA and responderB but
you don't want to couple the responder names to channel names you may provide additional mapping as such:

<!-- route the order to all responders--><si-xml:xpath-routerid="responderRouter"input-channel="orderChannel"><si-xml:xpath-expressionexpression="/request/responders"/><int-xml:mappingvalue="responderA"channel="channelA"/><int-xml:mappingvalue="responderB"channel="channelB"/></si-xml:xpath-router>

As we already said the default evaluation type for XPath expressions is NODESET which is converted
to a List<String> of channel names, thus handling single channel scenarios as well as multiple.
However certain XPath expressions may evaluate as String type from the very
beginning (e.g., 'name(./node())' - which will return the name of the root node) thus resulting in
the exception if default evaluation type (NODESET) is used.
For these scenarious you may use evaluate-as-string attribute which will allow you to manage the
evaluation type. It is FALSE by default, however if set to TRUE, the String evaluation type will be used.
For example if we want to route based on the name of the root node we can have use the following configuration:

The XPath Router does support custom implementations of the XmlPayloadConverter strategy, and when configuring
an 'xpath-router' element in XML, a reference to such an implementation may be provided via the 'converter' attribute.
However, the DefaultXmlPayloadConverter is used if this reference is not provided, and it
should be sufficient in most cases since it can convert from Node, Document, Source,
File, and String typed payloads. If you need to extend beyond the capabilities of
that default implementation, then an upstream Transformer is probably a better option
than providing a reference to a custom implementation of this strategy here.

24.6 Selecting xml messages using XPath

Two MessageSelector implementations are provided,
BooleanTestXPathMessageSelector and StringValueTestXPathMessageSelector.
BooleanTestXPathMessageSelector requires an XPathExpression which evaluates to a boolean,
for example boolean(/one/two) which will only select messages which have an element named
two which is a child of a root element named one. StringValueTestXPathMessageSelector
evaluates any XPath expression as a String and compares the result with the provided value.

<!-- Interceptor which rejects messages that do not have a root element order --><beanid="orderSelectingInterceptor"class="org.springframework.integration.channel.interceptor.MessageSelectingInterceptor"><constructor-arg><beanclass="org.springframework.integration.xml.selector.BooleanTestXPathMessageSelector"><constructor-argvalue="boolean(/order)" /></bean></constructor-arg></bean><!-- Interceptor which rejects messages that are not version one orders --><beanid="versionOneOrderSelectingInterceptor"class="org.springframework.integration.channel.interceptor.MessageSelectingInterceptor"><constructor-arg><beanclass="org.springframework.integration.xml.selector.StringValueTestXPathMessageSelector"><constructor-argvalue="/order/@version"index="0"/><constructor-argvalue="1"index="1"/></bean></constructor-arg></bean>

24.7 Transforming xml messages using XPath

When it comes to message transformation XPath is a great way to transform Messages that have XML
payloads by defining XPath transformers via <xpath-transformer/> element.

After sending this message to the 'inputChannel' the XPath transformer configured above will transform
this XML Message to a simple Message with payload of 'John Doe' all based on
the simple XPath Expression specified in the xpath-expression attribute.

XPath also has capability to perform simple conversion of extracted elements
to a desired type. Valid return types are defined in XPathConstants and follows
the conversion rules specified by the XPath.

The following constants are defined by the XPathConstants: BOOLEAN, DOM_OBJECT_MODEL, NODE, NODESET, NUMBER, STRING

You can configure the desired type by simply using evaluation-type
attribute of the <xpath-transformer/> element.

If you need to provide custom mapping for the node extracted by the XPath expression simply provide a reference to the
implementation of the org.springframework.xml.xpath.NodeMapper - an interface used by
XPathOperations implementations for mapping Node objects on a per-node basis. To provide a
reference to a NodeMapper simply use node-mapper attribute:

The DefaultXmlPayloadConverter is used if this reference is not provided, and it
should be sufficient in most cases since it can convert from Node, Document, Source,
File, and String typed payloads. If you need to extend beyond the capabilities of
that default implementation, then an upstream Transformer is probably a better option
than providing a reference to a custom implementation of this strategy here.

Combination of SpEL and XPath expressions

You can also combine Spring Expression Language (SpEL) expressions with XPath expression and configure
them using expression attribute:

xpath-expression id="testExpression" expression="/person/@age * 2"/>

In the above case the overall result of the expression will be the result of the XPath expression multiplied by 2.

24.8 XPath components namespace support

All XPath based components have namespace support allowing them to be configured as
Message Endpoints with the exception of the XPath selectors which are not designed to act as
endpoints. Each component allows the XPath to either be referenced at the top level or configured via a nested
<xpath-expression/> element. So the following configurations of an xpath-selector are all valid and represent the general
form of XPath namespace support. All forms of XPath expression result in the creation of an
XPathExpression using the Spring XPathExpressionFactory

XPath splitter namespace support allows the creation of a Message Endpoint with an input channel and output channel.

<!-- Split the order into items creating a new message for each item node --><si-xml:xpath-splitterid="orderItemSplitter"input-channel="orderChannel"output-channel="orderItemsChannel"><si-xml:xpath-expressionexpression="/order/items"/></si-xml:xpath-splitter><!-- Split the order into items creating a new document for each item--><si-xml:xpath-splitterid="orderItemDocumentSplitter"input-channel="orderChannel"output-channel="orderItemsChannel"create-documents="true"><si-xml:xpath-expressionexpression="/order/items"/><si:pollerfixed-rate="2000"/></si-xml:xpath-splitter>

25. XMPP Support

25.1 Introduction

XMPP describes a way for multiple agents to communicate with each other in a distributed system.
The canonical use case is to send and receive chat messages, though XMPP can be, and is, used for far more
applications.
XMPP is used to describe a network of actors. Within that network, actors may address each other directly, as well
as broadcast status changes (e.g. "presence").

XMPP provides the messaging fabric that underlies some of the biggest Instant Messaging networks in the world,
including Google Talk (GTalk) - which is also available from within GMail - and Facebook Chat.
There are many good open-source XMPP servers available. Two popular implementations are
Openfire
and
ejabberd

Spring integration provides support for XMPP via XMPP adapters which support sending and receiving both XMPP chat messages and
presence changes from other entries in your roster. As with other adapters, the XMPP adapters come with support for a
convenient namespace-based configuration.
To configure the XMPP namespace, include the following elements in the headers of your XML configuration file:

25.2 XMPP Connection

Before using inbound or outbound XMPP adapters to participate in the XMPP network, an actor must establish its XMPP connection. This
connection object could be shared by all XMPP adapters connected to a particular account. Typically this requires - at a minimum -
user, password, and host.
To create a basic XMPP connection, you can utilize the convenience of the namespace.

For added convenience you can rely on the default naming convention and omit the id attribute.
The default name xmppConnection will be used for this connection bean.

If the XMPP Connection goes stale, reconnection attempts will be made with an automatic login as long as the previous connection
state was logged (authenticated). We also register a ConnectionListener which will log connection events
if the DEBUG logging level is enabled.

25.3 XMPP Messages

25.3.1 Inbound Message Channel Adapter

The Spring Integration adapters support receiving chat messages from other users in the system. To do this, the
Inbound Message Channel Adapter "logs in" as a user on your behalf and receives the messages sent to that user. Those messages are then
forwarded to your Spring Integration client.
The payload of the inbound Spring Integration message may be of the raw type
org.jivesoftware.smack.packet.Message, or of the type
java.lang.String if you set the extract-payload attribute's value to 'true'
when configuring an adapter.
Configuration support for the XMPP Inbound Message Channel Adapter is provided via the inbound-channel-adapter element.

As you can see amongst the usual attributes this adapter also requires a reference to an XMPP Connection.

It is also important to mention that the XMPP inbound adapter is an event driven adapter
and a Lifecycle implementation.
When started it will register a PacketListener that will listen for incoming XMPP Chat Messages.
It forwards any received messages to the underlying adapter which will convert them to Spring Integration Messages and
send them to the specified channel. It will unregister the PacketListener
when it is stopped.

25.3.2 Outbound Message Channel Adapter

You may also send chat messages to other users on XMPP using the Outbound Message Channel Adapter.
Configuration support for the XMPP Outbound Message Channel Adapter is provided via the outbound-channel-adapter element.

The adapter expects as its input - at a minimum - a payload of type java.lang.String, and a header value
for XmppHeaders.CHAT_TO that specifies to which user the Message should be sent.
To create a message you might use the following Java code:

25.4 XMPP Presence

XMPP also supports broadcasting state. You can use this capability to
let people who have you on their roster see your state changes. This happens all the time with your IM clients; you
change your away status, and then set an away message, and everybody who has you on their roster sees your icon or username
change to reflect this new state, and additionally might see your new "away" message.
If you would like to receive notification, or notify others, of state changes, you can use Spring Integration's "presence" adapters.

25.4.1 Inbound Presence Message Channel Adapter

Spring Integration provides an Inbound Presence Message Channel Adapter which supports receiving Presence
events from other users in the system who happen to be on your Roster. To do this, the adapter "logs in" as a user
on your behalf, registers a RosterListener and forwards received Presence update events as Messages to the channel
identified by the channel attribute. The payload of the Message will be a org.jivesoftware.smack.packet.Presence
object (see http://www.igniterealtime.org/builds/smack/docs/3.1.0/javadoc/org/jivesoftware/smack/packet/Presence.html).

Configuration support for the XMPP Inbound Presence Message Channel Adapter is provided via
the presence-inbound-channel-adapter element.

As you can see amongst the usual attributes this adapter also requires a reference to an XMPP Connection.
It is also important to mention that this adapter is an event driven adapter and a Lifecycle implementation.
It will register a RosterListener when started and will unregister that RosterListener
when stopped.

25.4.2 Outbound Presence Message Channel Adapter

Spring Integration also supports sending Presence events to be seen by other users in the network who happen to have you on their
Roster. When you send a Message to the Outbound Presence Message Channel Adapter it extracts the payload,
which is expected to be of type org.jivesoftware.smack.packet.Presence
(see http://www.igniterealtime.org/builds/smack/docs/3.1.0/javadoc/org/jivesoftware/smack/packet/Presence.html) and sends it to
the XMPP Connection, thus advertising your presence events to the rest of the network.

Configuration support for the XMPP Outbound Presence Message Channel Adapter is provided via
the presence-outbound-channel-adapter element.

Like its inbound counterpart, it requires a reference to an XMPP Connection.

Note

If you are relying on the default naming convention for an XMPP Connection bean (described earlier), and you have only one
XMPP Connection bean configured in your Application Context, you may omit the xmpp-connection attribute.
In that case, the bean with the name xmppConnection will be located and injected into the adapter.

25.5 Appendices

Since Spring Integration XMPP support is based on the Smack 3.1 API (http://www.igniterealtime.org/downloads/index.jsp), it is important
to know a few details related to more complex configuration of the XMPP Connection object.

As stated earlier the xmpp-connection namespace support is designed to simplify basic connection configuration and
only supports a few common configuration attributes. However, the org.jivesoftware.smack.ConnectionConfiguration
object defines about 20 attributes, and there is no real value of adding namespace support for all of them. So, for more complex connection
configurations, simply configure an instance of our XmppConnectionFactoryBean as a regular bean, and inject a
org.jivesoftware.smack.ConnectionConfiguration as a constructor argument to that FactoryBean. Every property
you need, can be specified directly on that ConnectionConfiguration instance (a bean definition with the 'p' namespace would work well).
This way SSL, or any other attributes, could be set directly. Here's an example:

Another important aspect of the Smack API is static initializers. For more complex cases (e.g., registering a SASL Mechanism), you may need
to execute certain static initializers. One of those static initializers is SASLAuthentication, which allows
you to register supported SASL mechanisms. For that level of complexity, we would recommend Spring JavaConfig-style of the XMPP Connection
configuration. Then, you can configure the entire component through Java code and execute all other necessary Java code including
static initializers at the appropriate time.

For more information on the JavaConfig style of Application Context configuration, refer to the following section in the Spring Reference Manual:
http://static.springsource.org/spring/docs/3.0.x/spring-framework-reference/html/beans.html#beans-java

Part V. Appendices

Advanced Topics and Additional Resources

26. Message Publishing

The AOP Message Publishing feature allows you to construct and send a message as a by-product of a method invocation. For example, imagine you
have a component and every time the state of this component changes you would like to be notified via a Message. The easiest
way to send such notifications would be to send a message to a dedicated channel, but how would you connect the method invocation that
changes the state of the object to a message sending process, and how should the notification Message be structured?
The AOP Message Publishing feature handles these responsibilities with a configuration-driven approach.

26.1 Message Publishing Configuration

26.1.1 Annotation-driven approach via @Publisher annotation

The annotation-driven approach allows you to annotate any method with the @Publisher annotation,
specifying a 'channel' attribute.
The Message will be constructed from the return value of the method invocation and sent to a channel specified by the 'channel' attribute.
To further manage message structure, you can also use a combination of both @Payload
and @Header annotations.

Internally this message publishing feature of Spring Integration uses both Spring AOP by defining
PublisherAnnotationAdvisor and
Spring 3.0's Expression Language (SpEL) support, giving you considerable flexibility and control over the structure of the
Message it will publish.

The PublisherAnnotationAdvisor defines and binds the following variables:

#return - will bind to a return value allowing you to reference it or its
attributes (e.g., #return.foo where 'foo' is an attribute of the object bound to
#return)

#exception - will bind to an exception if one is thrown by the method invocation.

#args - will bind to method arguments, so individual arguments could be extracted by name
(e.g., #args.fname as in the above method)

In this example everything is the same as above, except that we are not using a default publishing channel. Instead we are specifying
the publishing channel via the 'channel' attribute of the @Publisher annotation.
We are also adding a @Header annotation which results in the Message header named 'last' having the same value
as the 'lname' method parameter. That header will be added to the newly constructed Message.

The above example is almost identical to the previous one. The only difference here is that we are using a @Payload annotation
on the method, thus explicitly specifying that the return value of the method should be used as the payload of the Message.

Here we are expanding on the previous configuration by using the Spring Expression Language in the
@Payload annotation to further instruct
the framework how the message should be constructed. In this particular case the message will be a concatenation of the return
value of the method invocation and the 'lname' input argument. The Message header named 'x' will have its value determined by
the 'num' input argument. That header will be added to the newly constructed Message.

You can instead use namespace support for a more concise configuration:

<si:annotation-configdefault-publisher-channel="defaultChannel"/>

Similar to other Spring annotations (@Component, @Scheduled, etc.), @Publisher can also be used as a meta-annotation.
That means you can define your own annotations
that will be treated in the same way as the @Publisher itself.

Here we defined the @Audit annotation which itself is annotated with @Publisher.
Also note that you can define a channel
attribute on the meta-annotation thus encapsulating the behavior of where messages will be sent inside of this annotation.
Now you can annotate any method:

@Auditpublic String test() {
return"foo";
}

In the above example every invocation of the test() method will result in a Message with a payload created from its return value.
Each Message will be sent to the channel named auditChannel. One of the benefits of this technique is that you can
avoid the duplication of the same channel name across multiple annotations. You also can provide a level of indirection between your own,
potentially domain-specific annotations and those provided by the framework.

You can also annotate the class which would mean that the properties of this annotation will be applied on every public method of that class.

26.1.2 XML-based approach via the <publishing-interceptor> element

The XML-based approach allows you to configure the same AOP-based Message Publishing functionality with
simple namespace-based configuration of a MessagePublishingInterceptor.
It certainly has some benefits over the annotation-driven approach since it
allows you to use AOP pointcut expressions, thus possibly intercepting multiple methods at once or
intercepting and publishing methods to which you don't have the source code.

To configure Message Publishing via XML, you only need to do the following two things:

As you can see the <publishing-interceptor> configuration looks rather similar to the Annotation-based approach,
and it also utilizes the power of the Spring 3.0 Expression Language.

In the above example the execution of the echo method of a testBean will
render a Message with the following structure:

The Message payload will be of type String with the content "Echoing: [value]" where value is the value
returned by an executed method.

The Message will have a header with the name "foo" and value "bar".

The Message will be sent to echoChannel.

The second method is very similar to the first. Here every method that begins with 'repl' will render a Message with the following structure:

The Message payload will be the same as in the above sample

The Message will have a header named "foo" whose value is the result of the SpEL expression 'bar'.toUpperCase() .

The Message will be sent to echoChannel.

The second method, mapping the execution of any method that begins with echoDef of testBean, will produce a
Message with the following structure.

The Message payload will be the value returned by an executed method.

Since the channel attribute is not provided explicitly, the Message will be sent to the
defaultChannel defined by the publisher.

For simple mapping rules you can rely on the publisher defaults. For example:

<publishing-interceptorid="anotherInterceptor"/>

This will map the return value of every method that matches the pointcut expression to a payload and will be sent to a default-channel.
If the defaultChannelis not specified (as above) the messages will be sent to the global nullChannel.

Async Publishing

One important thing to understand is that publishing occurs in the same thread as your component's execution. So by default in is synchronous.
This means that the entire message flow would have to wait until the publisher's flow completes.
However, quite often you want the complete opposite and that is to use this Message publishing feature to initiate asynchronous sub-flows.
For example, you might host a service (HTTP, WS etc.) which receives a remote request.You may want to send this request internally into a
process that might take a while. However you may also want to reply to the user right away. So, instead of sending inbound
requests for processing via the output channel (the conventional way), you can simply use 'output-channel' or a 'replyChannel' header
to send a simple acknowledgment-like reply back to the caller while using the Message publisher feature to initiate a complex flow.

EXAMPLE:
Here is the simple service that receives a complex payload, which needs to be sent further for processing, but it
also needs to reply to the caller with a simple acknowledgment.

public String echo(Object complexPayload) {
return"ACK";
}

So instead of hooking up the complex flow to the output channel we use the Message publishing feature instead. We configure it to create a
new Message using the input argument of the service method (above) and send that to the 'localProcessChannel'. And to make sure this sub-flow
is asynchronous all we need to do is send it to any type of asynchronous channel (ExecutorChannel in this example).

26.1.3 Producing and publishing messages based on a scheduled trigger

In the above sections we looked at the Message publishing feature of Spring Integration which constructs and publishes messages as by-products of Method invocations.
However in those cases, you are still responsible for invoking the method.
In Spring Integration 2.0 we've added another related useful feature: support for scheduled Message producers/publishers via the new "expression" attribute
on the 'inbound-channel-adapter' element. Scheduling could be based on several triggers, any one of which may be configured on the 'poller' sub-element.
Currently we support cron, fixed-rate, fixed-delay as well as any custom trigger implemented by you and
referenced by the 'trigger' attribute value.

As mentioned above, support for scheduled producers/publishers is provided via the <inbound-channel-adapter> xml element.
Let's look at couple of examples:

In the above example an inbound Channel Adapter will be created which will construct a Message with its payload being the result of the expression
defined in the expression attribute. Such messages will be created and sent every time the delay specified by the fixed-delay attribute occurs.

Here you can see that in a way very similar to the Message publishing feature we are enriching a newly constructed Message with
extra Message headers which can take scalar values or the results of evaluating Spring expressions.

If you need to implement your own custom trigger you can use the trigger attribute to provide a reference to any spring configured
bean which implements the org.springframework.scheduling.Trigger interface.

27. Transaction Support

27.1 Understanding Transactions in Message flows

Spring Integration exposes several hooks to address transactional needs of you message flows.
But to better understand these hooks and how you can benefit from them we must first revisit the 6 mechanisms
that could be used to initiate Message flows and see how transactional needs of these flows
could be addressed within each of these mechanisms.

Here are the 6 mechanisms to initiate a Message flow and their short summary (details for each are provided throughout this manual):

Scheduler - the way to initiate message flow based on scheduling events distributed
by a pre-configured Scheduler

Poller - similar to the Scheduler and is the way to initiate message flow based on scheduling
or interval-based events distributed by a pre-configured Poller

These 6 could be split in 2 general categories:

Message flows initiated by a USER process - Example scenarios in this category
would be invoking a Gateway method or explicitly sending a Message to a MessageChannel. In other words, these message flows depend on a third
party process (e.g., some code that we wrote) to be initiated.

Message flows initiated by a DAEMON process - Example scenarios in this category would be a Poller
polling a Message queue to initiate a new Message flow with the polled Message or a Scheduler scheduling the
process by creating a new Message and initiating a message flow at a predefined time.

Clearly the Gateway Proxy, MessageChannel.send(..) and MessagePublisher
all belong to the 1st category and Inbound Adapters/Gateways, Scheduler and Poller
belong to the 2nd.

So, how do we address transactional needs in various scenarios within each category and is there a need for Spring Integration
to provide something explicitly with regard to transactions for a particular scenario? Or, can Spring's Transaction Support be leveraged instead?.

The first and most obvious goal is NOT to re-invent something that has already been invented unless you can provide a better solution.
In our case Spring itself provides first class support for transaction management. So our goal here is not to provide something new but rather
delegate/use Spring to benefit from the existing support for transactions. In other words as a framework we must expose hooks to the Transaction management functionality
provided by Spring. But since Spring Integration configuration is based on Spring Configuration it is not always necessary to expose these hooks as they
are already exposed via Spring natively. Remember every Spring Integration component is a Spring Bean after all.

With this goal in mind let's look at the two scenarios.

If you think about it, Message flows that are initiated by the USER process (Category 1) and obviously configured in a Spring Application Context,
are subject to transactional configuration of such processes and therefore don't need to be explicitly configured by Spring Integration to support transactions.
The transaction could and should be initiated through standard Transaction support provided by Spring. The Spring Integration message flow will honor
the transactional semantics of the components naturally because it is Spring configured. For example, a Gateway or ServiceActivator method could
be annotated with @Transactional or TransactionInterceptor could be defined in an XML configuration
with a point-cut expression pointing to specific methods that should be transactional.
The bottom line is that you have full control over transaction configuration and boundaries in these scenarios.

However, things are a bit different when it comes to Message flows initiated by the DAEMON process (Category 2).
Although configured by the developer these flows do not directly involve a human or some other process to be initiated. These are trigger-based flows
that are initiated by a trigger process (DAEMON process) based on the configuration of such process. For example, we could have a Scheduler
initiating a message flow every Friday night of every week. We can also configure a trigger that initiates a Message flow every second, etc.
So, we obviously need a way to let these trigger-based processes know of our intention to make the resulting Message flows transactional so that
a Transaction context could be created whenever a new Message flow is initiated. In other words we need to expose some Transaction configuration, but ONLY enough
to delegate to Transaction support already provided by Spring (as we do in other scenarios).

Spring Integration provides transactional support for Pollers. Pollers are a special type of component because
we can call receive() within that poller task against a resource that is itself transactional thus including receive()
call in the the boundaries of the Transaction allowing it to be rolled back in case of a task failure. If we were to add the same support
for channels, the added transactions would affect all downstream components starting with that send() call. That is
providing a rather wide scope for transaction demarcation without any strong reason especially when Spring already provides several ways to
address the transactional needs of any component downstream. However the receive() method being included in a transaction
boundary is the "strong reason" for pollers.

27.1.1 Poller Transaction Support

Any time you configure a Poller you can provide transactional configuration via the transactional sub-element and its attributes:

As you can see this configuration looks very similar to native Spring transaction configuration. You must still provide a reference to a Transaction manager and specify
transaction attributes or rely on defaults (e.g., if the 'transaction-manager' attribute is not specified, it will default to the bean with the name 'transactionManager').
Internally the process would be wrapped in Spring's native Transaction where TransactionInterceptor is responsible for handling transactions.
For more information on how to configure a Transaction Manager, the types of Transaction Managers (e.g., JTA, Datasource etc.) and other details related to
transaction configuration please refer to Spring's Reference manual (Chapter 10 - Transaction Management).

With the above configuration all Message flows initiated by this poller will be transactional. For more information and details on a
Poller's transactional configuration please refer to section - 21.1.1. Polling and Transactions.

Along with transactions, several more cross cutting concerns might need to be addressed when running a Poller. To help with that,
the Poller element accepts an <advice-chain> sub-element which allows you to define a custom chain of Advice
instances to be applied on the Poller. (see section 4.4 for more details)
In Spring Integration 2.0, the Poller went through the a refactoring effort and is now using a proxy mechanism to address transactional
concerns as well as other cross cutting concerns. One of the significant changes evolving from this effort is that we
made <transactional> and <advice-chain> elements mutually exclusive.
The rationale behind this is that if you need more than one advice, and one of them is Transaction advice, then you can simply
include it in the <advice-chain> with the same convenience as before but with much more control
since you now have an option to position any advice in the desired order.

As yo can see from the example above, we have provided a very basic XML-based configuration of Spring Transaction advice - "txAdvice" and
included it within the <advice-chain> defined by the Poller.
If you only need to address transactional concerns of the Poller, then you can still use the <transactional> element
as a convinience.

27.2 Transaction Boundaries

Another important factor is the boundaries of Transactions within a Message flow.
When a transaction is started, the transaction context is bound to the current thread. So regardless of how many endpoints and channels you have in your
Message flow your transaction context will be preserved as long as you are ensuring that the flow continues on the same thread.
As soon as you break it by introducing a Pollable Channel or Executor Channel or initiate a new thread manually
in some service, the Transactional boundary will be broken as well. Essentially the Transaction will END right there, and if
a successful handoff has transpired between the threads, the flow would be considered a success and a COMMIT signal would be sent
even though the flow will continue and might still result in an Exception somewhere downstream. If such a flow were synchronous, that Exception could
be thrown back to the initiator of the Message flow who is also the initiator of the transactional context and the transaction would result in a ROLLBACK.
The middle ground is to use transactional channels at any point where a thread boundary is being broken. For example, you can use a Queue-backed Channel
that delegates to a transactional MessageStore strategy, or you could use a JMS-backed channel.

28. Security in Spring Integration

28.1 Introduction

Spring Integration builds upon the
Spring Security project
to enable role based security checks to be applied to channel send and receive invocations.

28.2 Securing channels

Spring Integration provides the interceptor ChannelSecurityInterceptor, which extends
AbstractSecurityInterceptor and intercepts send and receive calls on the channel. Access decisions
are then made with reference to a ChannelSecurityMetadataSource which provides the metadata describing
the send and receive access policies for certain channels. The interceptor requires that a valid SecurityContext
has been established by authenticating with Spring Security. See the Spring Security reference documentation for details.

Namespace support is provided to allow easy configuration of security constraints. This consists of the secured channels tag which allows
definition of one or more channel name patterns in conjunction with a definition of the security configuration for send and receive. The pattern
is a java.util.regexp.Pattern.

By default the secured-channels namespace element expects a bean named authenticationManager which implements
AuthenticationManager and a bean named accessDecisionManager which implements
AccessDecisionManager. Where this is not the case references to the appropriate beans can be configured
as attributes of the secured-channels element as below.

Appendix A. Spring Integration Samples

A.1 Introduction

As of Spring Integration 2.0, the samples are no longer included with the
Spring Integration distribution. Instead we have switched to a much simpler collaborative model that should promote
better community participation and, ideally, more contributions. Samples now have a dedicated Git repository and a
dedicated JIRA Issue Tracking system. Sample development will also have its own lifecycle which is not dependent on the
lifecycle of the framework releases although the repository will still be tagged with each major release for compatibility
reasons.

The great benefit to the community is that we can now add more samples and make them available to you right away
without waiting for the next release. Having its own JIRA that is not tied to the the actual
framework is also a great benefit. You now have a dedicated place to suggest samples as well as report issues with existing
samples. Or, you may want to submit a sample to us as an attachment through the JIRA or, better, through
the collaborative model that Git promotes. If we believe your sample adds value, we
would be more then glad to add it to the 'samples' repository, properly crediting you as the author.

A.2 Where to get Samples

To monitor samples development and to get more information on the repository you can visit the following
URL: http://git.springsource.org/spring-integration/samples
Since we are using Git as the SCM, we should use the proper terminology as well when it comes to the tasks you need to perform to make
samples available locally on your machine. For more information on Git SCM please visit their
website: http://git-scm.com/

CLONE samples repository. (For those unfamiliar with Git, this is somewhat the equivalent of a checkout.)

This is the first step you should go through. You must have Git installed on your machine. There are many GUI-based products
available for many platforms. A simple Google search will help you find them.
To clone the Spring Integration samples repository, issue the following at the command line:

That is all you need to do. Now you have cloned the entire samples repository. Since the samples repository is a live
repository, you might want to perform periodic "pulls" to get new samples as well as updates to the existing samples.
To get the updates use the git PULL command:

A.3 Samples Structure

The structure of the samples changed as well. With plans for more samples we realized that some
samples have different goals than others. While they all share the common goal of showing you how to apply and work with
the Spring Integration framework, they also differ in areas where some samples are meant to concentrate on a technical
use case while others focus on a business use case, and some samples are all about showcasing various techniques that
could be applied to address certain scenarios (both technical and business). The new categorization of samples will allow us
to better organize them based on the problem each sample addresses while giving you a simpler way of finding the right sample
for your needs.

Currently there are 4 categories. Within the samples repository each category has its own directory which is named after the
category name:

BASIC (samples/basic)

This is a good place to get started. The samples here are technically motivated and demonstrate the bare
minimum with regard to configuration and code. These should help you to get started quickly by introducing you to the basic concepts,
API and configuration of Spring Integration as well as Enterprise Integration Patterns (EIP). For example, if you are
looking for an answer on how to implement and wire a Service Activator to a Message Channel
or how to use a Messaging Gateway as a facade to your message exchange, or how to get started with using MAIL or
TCP/UDP modules etc., this would be the right place to find a good sample. The bottom line is this is a good place
to get started.

INTERMEDIATE (samples/intermediate)

This category targets developers who are already familiar with the Spring Integration framework (past getting started),
but need some more guidance while resolving the more advanced technical problems one might deal with
after switching to a Messaging architecture.
For example, if you are looking for an answer on how to handle errors in various message exchange
scenarios or how to properly configure the Aggregator for the situations where some messages
might not ever arrive for aggregation, or any other issue that goes beyond a basic implementation and configuration
of a particular component and addresses what else types of problems, this
would be the right place to find these type of samples.

ADVANCED (samples/advanced)

This category targets developers who are very familiar with the Spring Integration framework but are looking to
extend it to address a specific custom need by using Spring Integration's public API.
For example, if you are looking for samples showing you how to implement a custom Channel or
Consumer (event-based or polling-based), or you are trying to figure out what is the most appropriate
way to implement a custom Bean parser on top of the Spring Integration Bean parser hierarchy when implementing your own
namespace and schema for a custom component, this would be the right place to look.
Here you can also find samples that will help you with Adapter development. Spring Integration comes
with an extensive library of adapters to allow you to connect remote systems with the Spring Integration messaging framework.
However you might have a need to integrate with a system for which the core framework does not provide an adapter.
So, you may decide to implement your own (and potentially contribute it). This category would include samples showing you how.

APPLICATIONS (samples/applications)

This category targets developers and architects who have a good understanding of Message-driven architecture and
EIP, and an above average understanding of Spring and Spring Integration who are looking for samples that
address a particular business problem. In other words the emphasis of samples in this category
is business use cases and how they can be solved with a Message-Driven Architecture and Spring Integration
in particular.
For example, if you are interested to see how a Loan Broker or Travel Agent
process could be implemented and automated via Spring Integration, this would be the right place to find these types of samples.

Important

Remember: Spring Integration is a community driven framework, therefore community participation is IMPORTANT.
That includes Samples; so, if you can't find what you are looking for, let us know!

A.4 Samples

Currently Spring Integration comes with quite a few samples and you can only expect more.
To help you better navigate through them, each sample comes with its own readme.txt file which covers
several details about the sample (e.g., what EIP patterns it addresses, what problem it is trying to solve, how to run sample etc.).
However, certain samples require a more detailed and sometimes graphical explanation. In this section you'll
find details on samples that we believe require special attention.

A.4.1 Loan Broker

In this section, we will review the Loan Broker sample application that is included in the
Spring Integration samples. This sample is inspired by one of the samples featured in Gregor
Hohpe and Bobby Woolf's book, Enterprise Integration Patterns.

The diagram below represents the entire process

Now lets look at this process in more detail

At the core of an EIP architecture are the very simple yet powerful concepts of Pipes and Filters, and of course: Messages.
Endpoints (Filters) are connected with one another via Channels (Pipes). The producing endpoint sends Message to the Channel,
and the Message is retrieved
by the Consuming endpoint. This architecture is meant to define various mechanisms that describe HOW information is exchanged between
the endpoints, without any awareness of WHAT those endpoints are or what information they are exchanging. Thus, it provides for a very loosely
coupled and flexible collaboration model while also decoupling Integration concerns from Business concerns. EIP extends this architecture
by further defining:

The core filters and patterns around how filters collaborate with pipes
(Message Router, Splitters and Aggregators, various Message Transformation patterns, etc.)

The details and variations of this use case are very nicely described in Chapter 9 of the EIP Book, but here is the brief summary;
A Consumer while shopping for the best Loan Quote(s) subscribes to the services of a Loan Broker, which handles details such as:

Determine the most appropriate Banks (e.g., based on consumer's credit history/score)

Send a Loan quote request to each selected Bank

Collect responses from each Bank

Filter responses and determine the best quote(s), based on consumer's requirements.

Pass the Loan quote(s) back to the consumer.

Obviously the real process of obtaining a loan quote is a bit more complex, but since our goal here is to demonstrate how
Enterprise Integration Patterns are realized and implemented within SI, the use case has been simplified to concentrate only on
the Integration aspects of the process. It is not an attempt to give you an advice in consumer finances.

As you can see, by hiring a Loan Broker, the consumer is isolated from the details of the Loan Broker's operations, and each Loan Broker's
operations may defer from one another to maintain competitive advantage, so whatever we assemble/implement must be flexible so any changes
could be introduced quickly and painlessly.
Speaking of change, the Loan Broker sample does not actually talk to any 'imaginary' Banks or Credit bureaus. Those services are stubbed out.
Our goal here is to assemble, orchestrate and test the integration aspect of the process as a whole. Only then can we start thinking about
wiring such process to the real services. At that time the assembled process and its configuration will not change regardless of the number
of Banks a particular Loan Broker is dealing with, or the type of communication media (or protocols) used (JMS, WS, TCP, etc.)
to communicate with these Banks.

DESIGN

As you analyze the 6 requirements above you'll quickly see that they all fall into the category of Integration concerns.
For example, in the consumer pre-screening step we need to gather additional information about the consumer and the consumer's desires
and enrich the loan request with additional meta information. We then have to filter such information to select the most appropriate list of
Banks, and so on. Enrich, filter, select – these are all integration concerns for which EIP defines a solution in the form of patterns.
SI provides an implementation of these patterns.

Messaging Gateway

The Messaging Gateway pattern provides a simple mechanism to access messaging systems, including our Loan Broker.
In SI you define the Gateway as a Plain Old Java Interface (no need to provide an implementation), configure it via the
XML <gateway> element or via annotation and use it as any other Spring bean. SI will take care of
delegating and mapping method invocations to the Messaging infrastructure by generating a Message (payload is mapped to an
input parameter of the method) and sending it to the designated channel.

Our current Gateway provides two methods that could be invoked. One that will return the best single quote and another one that
will return all quotes. Somehow downstream we need to know what type of reply the caller is looking for. The best way to achieve
this in Messaging architecture is to enrich the content of the message with some meta-data describing your intentions.
Content Enricher is one of the patterns that addresses this and although Spring Integration does provide a
separate configuration element to enrich Message Headers with arbitrary data (we'll see it later), as a convenience, since
Gateway element is responsible to construct the initial Message it provides embedded
capability to enrich the newly created Message with arbitrary Message Headers. In our
example we are adding header RESPONSE_TYPE with value 'BEST'' whenever the getBestQuote() method is invoked. For other method
we are not adding any header. Now we can check downstream for an existence of this header and based on its presence and its value
we can determine what type of reply the caller is looking for.

Based on the use case we also know there are some pre-screening steps that needs to be performed such as getting and evaluating the consumer's
credit score, simply because some premiere Banks will only typically accept quote requests from consumers that meet a minimum credit
score requirement. So it would be nice if the Message would be enriched with such information before it is forwarded
to the Banks. It would also be nice if when several processes needs to be completed to provide such meta-information, those
processes could be grouped in a single unit. In our use case we need to determine credit score and based on the credit score and some
rule select a list of Message Channels (Bank Channels) we will sent quote request to.

As you can see from the above configuration we have a chain with inner header-enricher element which will further enrich the
content of the Message with the header CREDIT_SCORE and value that will be determined by the call to a
credit service (simple POJO spring bean identified by 'creditBureau' name) and then it will delegate to the Message Router

Message Router

There are several implementations of the Message Routing pattern available in Spring Integration. Here we are using a
router that will determine a list of channels based on evaluating an expression (Spring Expression Language) which will look at
the credit score that was determined is the previous step and will select the list of channels from the Map bean with id 'banks'
whose values are 'premier' or 'secondary' based o the value of credit score. Once the list of Channels is selected, the
Message will be routed to those Channels.

Now, one last thing the Loan Broker needs to to is to receive the loan quotes form the banks, aggregate them by consumer
(we don't want to show quotes from one consumer to another), assemble the response based on the consumer's selection criteria
(single best quote or all quotes) and reply back to the consumer.

Message Aggregator

An Aggregator pattern describes an endpoint which groups related Messages into a single
Message. Criteria and rules can be provided to determine an aggregation and correlation strategy.
SI provides several implementations of the Aggregator pattern as well as a convenient name-space based configuration.

Our Loan Broker defines a 'quotesAggregator' bean via the <aggregator> element which provides a default
aggregation and correlation strategy. The default correlation strategy correlates messages based on the correlationId header
(see Correlation Identifier pattern). What's interesting is that we never provided the value for this header.
It was set earlier by the router automatically, when it generated a separate Message for each Bank channel.

Once the Messages are correlated they are released to the actual Aggregator implementation.
Although default Aggregator is provided by SI, its strategy (gather the list of payloads from all
Messages and construct a new Message with this List as payload) does not satisfy our
requirement. The reason is that our consumer might require a single best quote or all quotes. To communicate the consumer's
intention, earlier in the process we set the RESPONSE_TYPE header. Now we have to evaluate this header and return either
all the quotes (the default aggregation strategy would work) or the best quote (the default aggregation strategy will not work
because we have to determine which loan quote is the best).

Obviously selecting the best quote could be based on complex criteria and would influence the complexity of the aggregator implementation and
configuration, but for now we are making it simple. If consumer wants the best quote we will select a quote with the lowest interest
rate. To accomplish that the LoanQuoteAggregator.java will sort all the quotes and return the first one.
The LoanQuote.java implements Comparable which compares quotes based on the rate attribute.
Once the response Message is created it is sent to the default-reply-channel of the Messaging Gateway
(thus the consumer) which started the process. Our consumer got the Loan Quote!

Conclusion

As you can see a rather complex process was assembled based on POJO (read existing, legacy), light weight, embeddable messaging
framework (Spring Integration) with a loosely coupled programming model intended to simplify integration of heterogeneous systems
without requiring a heavy-weight ESB-like engine or proprietary development and deployment environment, because as a developer you
should not be porting your Swing or console-based application to an ESB-like server or implementing proprietary interfaces just
because you have an integration concern.

This and other samples in this section are build on top of Enterprise Integration Patterns that meant to describe "building blocks"
for YOUR solution but not to be solutions in of themselves. Integration concerns exist in all types of applications (server based and not)
and should not require change in design, testing and deployment strategy if such applications need to integrate with one another.

A.4.2 The Cafe Sample

In this section, we will review a Cafe sample application that is included in the
Spring Integration samples. This sample is inspired by another sample featured in Gregor
Hohpe's Ramblings.

The domain is that of a Cafe, and the basic flow is depicted in the following diagram:

The Order object may contain multiple OrderItems. Once the order
is placed, a Splitter will break the composite order message into a single message per
drink. Each of these is then processed by a Router that determines whether the drink is hot
or cold (checking the OrderItem object's 'isIced' property). The
Barista prepares each drink, but hot and cold drink preparation are handled by two
distinct methods: 'prepareHotDrink' and 'prepareColdDrink'. The prepared drinks are then sent to the Waiter where
they are aggregated into a Delivery object.

As you can see, each Message Endpoint is connected to input and/or output channels. Each endpoint will manage
its own Lifecycle (by default endpoints start automatically upon initialization - to prevent that add the
"auto-startup" attribute with a value of "false"). Most importantly, notice that the objects are simple POJOs
with strongly typed method arguments. For example, here is the Splitter:

In the case of the Router, the return value does not have to be a MessageChannel
instance (although it can be). As you see in this example, a String-value representing the channel name is
returned instead.

Now turning back to the XML, you see that there are two <service-activator> elements. Each of these
is delegating to the same Barista instance but different methods: 'prepareHotDrink'
or 'prepareColdDrink' corresponding to the two channels where order items have been routed.

As you can see from the code excerpt above, the barista methods have different delays (the hot drinks take 5
times as long to prepare). This simulates work being completed at different rates. When the
CafeDemo 'main' method runs, it will loop 100 times sending a single hot drink and a
single cold drink each time. It actually sends the messages by invoking the 'placeOrder' method on the Cafe
interface. Above, you will see that the <gateway> element is specified in the configuration file. This
triggers the creation of a proxy that implements the given 'service-interface' and connects it to a channel.
The channel name is provided on the @Gateway annotation of the Cafe interface.

To run this sample as well as 8 others, refer to the README.txt within the "samples" directory
of the main distribution as described at the beginning of this chapter.

When you run cafeDemo, you will see that the cold drinks are initially prepared more quickly than the hot drinks.
Because there is an aggregator, the cold drinks are effectively limited by the rate of the hot drink preparation.
This is to be expected based on their respective delays of 1000 and 5000 milliseconds. However, by configuring a
poller with a concurrent task executor, you can dramatically change the results. For example, you could use a
thread pool executor with 5 workers for the hot drink barista while keeping the cold drink barista as it is:

Also, notice that the worker thread name is displayed with each invocation. You will see that the hot drinks are
prepared by the task-executor threads. If you provide a much shorter poller interval (such as 100 milliseconds),
then you will notice that occasionally it throttles the input by forcing the task-scheduler (the caller) to invoke
the operation.

Note

In addition to experimenting with the poller's concurrency settings, you can also add the 'transactional'
sub-element and then refer to any PlatformTransactionManager instance within the context.

A.4.3 The XML Messaging Sample

The xml messaging sample in the org.springframework.integration.samples.xml illustrates how to use
some of the provided components which deal with xml payloads. The sample uses the idea of processing an order for books
represented as xml.

First the order is split into a number of messages, each one representing a single order item using
the XPath splitter component.

A service activator is then used to pass the message into a stock checker POJO. The order item document is enriched with information
from the stock checker about order item stock level. This enriched order item message is then used to route the message. In the
case where the order item is in stock the message is routed to the warehouse. The XPath router makes use of the default
ChannelResolver strategy which maps the XPath evaluation result to a channel reference in the ApplicationContext.

Appendix B. Configuration

B.1 Introduction

Spring Integration offers a number of configuration options. Which option you choose depends upon your particular
needs and at what level you prefer to work. As with the Spring framework in general, it is also possible to mix
and match the various techniques according to the particular problem at hand. For example, you may choose the
XSD-based namespace for the majority of configuration combined with a handful of objects that are configured with
annotations. As much as possible, the two provide consistent naming. XML elements defined by the XSD schema will
match the names of annotations, and the attributes of those XML elements will match the names of annotation
properties. Direct usage of the API is of course always an option, but we expect that most users will choose one
of the higher-level options, or a combination of the namespace-based and annotation-driven configuration.

B.2 Namespace Support

Spring Integration components can be configured with XML elements that map directly to the terminology and
concepts of enterprise integration. In many cases, the element names match those of the
Enterprise Integration Patterns.

You can choose any name after "xmlns:"; integration is used here for clarity, but you might
prefer a shorter abbreviation. Of course if you are using an XML-editor or IDE support, then the availability of
auto-completion may convince you to keep the longer name for clarity. Alternatively, you can create configuration
files that use the Spring Integration schema as the primary namespace:

When using this alternative, no prefix is necessary for the Spring Integration elements. On the other hand, if
you want to define a generic Spring "bean" within the same configuration file, then a prefix would be required
for the bean element (<beans:bean ... />). Since it is generally a good idea to modularize the
configuration files themselves based on responsibility and/or architectural layer, you may find it appropriate to
use the latter approach in the integration-focused configuration files, since generic beans are seldom necessary
within those same files. For purposes of this documentation, we will assume the "integration" namespace is
primary.

Many other namespaces are provided within the Spring Integration distribution. In fact, each adapter type (JMS,
File, etc.) that provides namespace support defines its elements within a separate schema. In order to use these
elements, simply add the necessary namespaces with an "xmlns" entry and the corresponding "schemaLocation" mapping.
For example, the following root element shows several of these namespace declarations:

The reference manual provides specific examples of the various elements in their corresponding chapters. Here, the
main thing to recognize is the consistency of the naming for each namespace URI and schema location.

B.3 Configuring the Task Scheduler

In Spring Integration, the ApplicationContext plays the central role of a Message Bus, and there are only a
couple configuration options to consider. First, you may want to control the central TaskScheduler instance.
You can do so by providing a single bean with the name "taskScheduler". This is also defined as a constant:

IntegrationContextUtils.TASK_SCHEDULER_BEAN_NAME

By default Spring Integration relies on an instance of ThreadPoolTaskScheduler as described in the Task Execution and Scheduling
section of the Spring Framework reference manual. That default TaskScheduler will startup automatically with a
pool of 10 threads. If you provide your own TaskScheduler instance instead, you can set the 'autoStartup' property
to false, and/or you can provide your own pool size value.

When Polling Consumers provide an explicit task-executor reference in their configuration, the invocation of
the handler methods will happen within that executor's thread pool and not the main scheduler pool. However,
when no task-executor is provided for an endpoint's poller, it will be invoked by one of the main scheduler's
threads.

Note

An endpoint is a Polling Consumer if its input channel is one of the queue-based
(i.e. pollable) channels. On the other hand, Event Driven Consumers are those whose
input channels have dispatchers instead of queues (i.e. they are subscribable). Such endpoints have no
poller configuration since their handlers will be invoked directly.

The next section will describe what happens if Exceptions occur within the asynchronous invocations.

B.4 Error Handling

As described in the overview at the very beginning of this manual, one of the main motivations behind a
Message-oriented framework like Spring Integration is to promote loose-coupling between components. The
Message Channel plays an important role in that producers and consumers do not have to know about each
other. However, the advantages also have some drawbacks. Some things become more complicated in a very
loosely coupled environment, and one example is error handling.

When sending a Message to a channel, the component that ultimately handles that Message may or may not
be operating within the same thread as the sender. If using a simple default DirectChannel (with the
<channel> element that has no <queue> sub-element and no 'task-executor' attribute), the
Message-handling will occur in the same thread as the Message-sending. In that case, if an Exception
is thrown, it can be caught by the sender (or it may propagate past the sender if it is an uncaught
RuntimeException). So far, everything is fine. This is the same behavior as an Exception-throwing
operation in a normal call stack. However, when adding the asynchronous aspect, things become much
more complicated. For instance, if the 'channel' element does provide a 'queue'
sub-element, then the component that handles the Message will be operating in a
different thread than the sender. The sender may have dropped the Message into the channel and moved
on to other things. There is no way for the Exception to be thrown directly back to that sender using
standard Exception throwing techniques. Instead, to handle errors for asynchronous processes requires
an asynchronous error-handling mechanism as well.

Spring Integration supports error handling for its components by publishing errors to a Message Channel.
Specifically, the Exception will become the payload of a Spring Integration Message. That Message will
then be sent to a Message Channel that is resolved in a way that is similar to the 'replyChannel'
resolution. First, if the request Message being handled at the time the Exception occurred contains
an 'errorChannel' header (the header name is defined in the constant: MessageHeaders.ERROR_CHANNEL),
the ErrorMessage will be sent to that channel. Otherwise, the error handler will send to a "global"
channel whose bean name is "errorChannel" (this is also defined as a constant:
IntegrationContextUtils.ERROR_CHANNEL_BEAN_NAME).

Whenever relying on Spring Integration's XML namespace support, a default "errorChannel" bean will be
created behind the scenes. However, you can just as easily define your own if you want to control the
settings.

<channelid="errorChannel"><queuecapacity="500"/></channel>

Note

The default "errorChannel" is a PublishSubscribeChannel.

The most important thing to understand here is that the messaging-based error handling will only apply
to Exceptions that are thrown by a Spring Integration task that is executing within a TaskExecutor.
This does not apply to Exceptions thrown by a handler that is operating within
the same thread as the sender (e.g. through a DirectChannel as described above).

Note

When Exceptions occur in a scheduled poller task's execution, those exceptions will be wrapped in
ErrorMessages and sent to the 'errorChannel' as well.

To enable global error handling, simply register a handler on that channel. For example, you can configure
Spring Integration's ErrorMessageExceptionTypeRouter as the handler of an endpoint
that is subscribed to the 'errorChannel'. That router can then spread the error messages across multiple
channels based on Exception type.

B.5 Annotation Support

In addition to the XML namespace support for configuring Message Endpoints, it is also possible to use
annotations. First, Spring Integration provides the class-level @MessageEndpoint
as a stereotype annotation meaning that is itself annotated with Spring's @Component
annotation and therefore is recognized automatically as a bean definition when using Spring component-scanning.

Even more importantly are the various Method-level annotations that indicate the annotated method is capable of
handling a message. The following example demonstrates both:

Exactly what it means for the method to "handle" the Message depends on the particular annotation. The following
are available with Spring Integration, and the behavior of each is described in its own chapter or section within
this reference: @Transformer, @Router, @Splitter, @Aggregator, @ServiceActivator, and @ChannelAdapter.

Note

The @MessageEndpoint is not required if using XML configuration in combination with annotations. If you want to
configure a POJO reference from the "ref" attribute of a <service-activator/> element, it is sufficient to
provide the method-level annotations. In that case, the annotation prevents ambiguity even when no "method"
attribute exists on the <service-activator/> element.

In most cases, the annotated handler method should not require the Message type as its
parameter. Instead, the method parameter type can match the message's payload type.

When the method parameter should be mapped from a value in the MessageHeaders, another
option is to use the parameter-level @Header annotation. In general, methods
annotated with the Spring Integration annotations can either accept the Message itself, the
message payload, or a header value (with @Header) as the parameter. In fact, the method can accept a combination,
such as:

The value of the annotation can also be a SpEL expression (e.g., 'payload.getCustomerId()') which is quite useful when
the name of the header has to be dynamically computed. It also provides an optional 'required' property which
specifies whether the attribute value must be available within the header. The default value for 'required' is true.

For several of these annotations, when a Message-handling method returns a non-null value, the endpoint will
attempt to send a reply. This is consistent across both configuration options (namespace and annotations) in
that such an endpoint's output channel will be used if available, and the REPLY_CHANNEL message header value
will be used as a fallback.

Tip

The combination of output channels on endpoints and the reply channel message header enables a pipeline approach
where multiple components have an output channel, and the final component simply allows the reply message to be
forwarded to the reply channel as specified in the original request message. In other words, the final component
depends on the information provided by the original sender and can dynamically support any number of clients as a
result. This is an example of Return Address.

In addition to the examples shown here, these annotations also support inputChannel and outputChannel properties.

That provides a pure annotation-driven alternative to the XML configuration. However, it is generally recommended
to use XML for the endpoints, since it is easier to keep track of the overall configuration in a single, external
location (and besides the namespace-based XML configuration is not very verbose). If you do prefer to provide
channels with the annotations however, you just need to enable a SI Annotations BeanPostProcessor. The following element should
be added:

<int:annotation-config/>

Note

When configuring the "inputChannel" and "outputChannel" with annotations, the "inputChannel"
must be a reference to a SubscribableChannel instance.
Otherwise, it would be necessary to also provide the full poller configuration via annotations, and those
settings (e.g. the trigger for scheduling the poller) should be externalized rather than hard-coded within
an annotation. If the input channel that you want to receive Messages from is indeed a
PollableChannel instance, one option to consider is the Messaging Bridge.
Spring Integration's "bridge" element can be used to connect a PollableChannel directly to a
SubscribableChannel. Then, the polling metadata is externally configured, but the annotation option is
still available. For more detail see Section 3.3, “Messaging Bridge”.

B.6 Message Mapping rules and conventions

Spring Integration implements a flexible facility to map Messages to Methods and their arguments without
providing extra configuration by relying on some default rules as well as defining certain conventions.

B.6.1 Simple Scenarios

Single un-annotated parameter (object or primitive) which is not a Map/Properties with non-void return type;

public String foo(Object o);

Details:

Input parameter is Message Payload. If parameter type is not compatible with Message Payload an
attempt will be made to convert it using Conversion Service provided by Spring 3.0. The return value
will be incorporated as a Payload of the returned Message

Single un-annotated parameter (object or primitive) which is not a Map/Properties with Message return type;

public Message foo(Object o);

Details:

Input parameter is Message Payload. If parameter type is not compatible with Message Payload an attempt
will be made to convert it using Conversion Service provided by Spring 3.0. The return value is a newly constructed
Message that will be sent to the next destination.

Single parameter which is a Message or its subclass with arbitrary object/primitive return type;

publicint foo(Message msg);

Details:

Input parameter is Message itself. The return value will become a payload of the
Message that will be sent to the next destination.

Single parameter which is a Message or its subclass with Message or its subclass as a return type;

public Message foo(Message msg);

Details:

Input parameter is Message itself. The return value is a newly constructed Message that will be sent to the next destination.

Single parameter which is of type Map or Properties with Message as a return type;

public Message foo(Map m);

Details:

This one is a bit interesting. Although at first it might seem like an easy mapping straight to Message Headers,
the preference is always given to a Message Payload. This means that if Message Payload is of type Map, this input argument will
represent Message Payload. However if Message Payload is not of type Map, then no conversion via Conversion Service will be
attempted and the input argument will be mapped to Message Headers.

Two parameters where one of them is arbitrary non-Map/Properties type object/primitive and another is Map/Properties type object (regardless of the return)

public Message foo(Map h, <T> t);

Details:

This combination contains two input parameters where one of them is of type Map. Naturally the non-Map parameters (regardless of the order) will
be mapped to a Message Payload and the Map/Properties (regardless of the order) will be mapped to Message Headers giving you a nice POJO way
of interacting with Message structure.

No parameters (regardless of the return)

public String foo();

Details:

This Message Handler method will be invoked based on the Message sent to the input channel this handler is hooked up to,
however no Message data will be mapped, thus making Message act as event/trigger to invoke such handlerThe output will be
mapped according to the rules above

No parameters, void return

publicvoid foo();

Details:

Same as above, but no output

Annotation based mappings

Annotation based mapping is the safest and least ambiguous approach to map Messages to Methods. There wil be many pointers to annotation
based mapping throughout this manual, however here are couple of examples:

public String foo(@Payload String s, @Header("foo") String b)

Very simple and explicit way of mapping Messages to method. As you'll see later on, without an annotation this signature
would result in an ambiguous condition. However by explicitly mapping the first argument to a Message Payload and the second argument to
a value of the 'foo' Message Header, we have avoided any ambiguity.

public String foo(@Payload String s, @RequestParam("foo") String b)

Looks almost identical to the previous example, however @RequestMapping or any other non-Spring Integration mapping annotation
is irrelevant and therefore will be ignored leaving the second parameter unmapped. Although the second parameter could
easily be mapped to a Payload, there can only be one Payload. Therefore this method mapping is ambiguous.

public String foo(String s, @Header("foo") String b)

The same as above. The only difference is that the first argument will be mapped to the Message Payload implicitly.

Yet another signature that would definitely be treated as ambiguous without annotations because it has more than 2 arguments.
Furthermore, two of them are Maps. However, with annotation-based mapping, the ambiguity is easily avoided. In this example
the first argument is mapped to all the Message Headers, while the second and third argument map to the values of Message Headers
'foo' and 'bar'. The payload is not being mapped to any argument.

B.6.2 Complex Scenarios

Multiple parameters:

Multiple parameters could create a lot of ambiguity with regards to determining the appropriate mappings. The general advice is to annotate your method parameters with @Payload and/or @Header/@Headers
Below are some of the examples of ambiguous conditions which result in an Exception being raised.

public String foo(String s, int i)

- the two parameters are equal in weight, therefore there is no way to determine which one is a payload.

public String foo(String s, Map m, String b)

- almost the same as above. Although the Map could be easily mapped to Message Headers, there is no way to determine what to do with the two Strings.

public String foo(Map m, Map f)

- although one might argue that one Map could be mapped to Message Payload and another one to Message Headers, it would be unreasonable to rely on the order (e.g., first is Payload, second Headers)

Tip

Basically any method signature with more than one method argument which is not (Map, <T>), and those parameters are not annotated, will result in an ambiguous condition thus triggering an Exception.

Multiple methods:

Message Handlers with multiple methods are mapped based on the same rules that are described above, however some scenarios might still look confusing.

Multiple methods (same or different name) with legal (mappable) signatures:

As you can see, the Message could be mapped to either method. The first method would be invoked where Message Payload
could be mapped to 'str' and Message Headers could be mapped to 'm'. The second method could easily also be a candidate where
only Message Headers are mapped to 'm'. To make meters worse both methods have the same name which at first might look very
ambiguous considering the following configuration:

At this point it would be important to understand Spring Integration mapping Conventions where at the very core,
mappings are based on Payload first and everything else next. In other words the method whose argument could be mapped
to a Payload will take precedence over all other methods.

If you look at it you can probably see a truly ambiguous condition. In this example since both methods have signatures that
could be mapped to a Message Payload. They also have the same name. Such handler methods will trigger an Exception.
However if the method names were different you could influence the mapping with a 'method' attribute (see below):

Now there is no ambiguity since the configuration explicitly maps to the 'bar' method which has no name conflicts.

Appendix C. Additional Resources

C.1 Spring Integration Home

The definitive source of information about Spring Integration is the
Spring Integration Home at
http://www.springsource.org. That site serves as a hub of
information and is the best place to find up-to-date announcements about the project as well as links to
articles, blogs, and new sample applications.